[iOS WebKit2] Make back-buffers purgeable when possible

[iOS WebKit2] Make back-buffers purgeable when possible
https://bugs.webkit.org/show_bug.cgi?id=131213
<rdar://problem/15373906>

Reviewed by Simon Fraser.

We should make back-buffers volatile when their owning
layer falls out of use (either stops being repainted or
is unparented), as they are inherently recreatable.

* Shared/mac/RemoteLayerBackingStore.h:
(WebKit::RemoteLayerBackingStore::volatility):
(WebKit::RemoteLayerBackingStore::lastDisplayTime):
Add storage and a getter for the backing store's last paint time.
Add setter/getter for volatility of backing store.
Add RemoteLayerTreeContext param to constructor.

* Shared/mac/RemoteLayerBackingStore.mm:
(WebKit::RemoteLayerBackingStore::RemoteLayerBackingStore):
(WebKit::RemoteLayerBackingStore::~RemoteLayerBackingStore):
Inform the RemoteLayerTreeContext when backing store
is created and destroyed. Since we only have a Context in
the Web process, this will only happen there.

(WebKit::RemoteLayerBackingStore::encode):
(WebKit::RemoteLayerBackingStore::decode):
Move some assertions out of #else blocks which are preceded by returns.

(WebKit::RemoteLayerBackingStore::display):
Update the last display time.
If the backing store was volatile, make it non-volatile.

(WebKit::RemoteLayerBackingStore::setVolatility):
Mark the relevant surfaces as volatile or not.
setVolatility:
    - fails and returns false if any surfaces weren't marked
      volatile because they were in use
    - marks the whole backing store for repaint if the front
      surface was purged

* Shared/mac/RemoteLayerTreeTransaction.mm:
(WebKit::RemoteLayerTreeTransaction::LayerProperties::encode):
(WebKit::RemoteLayerTreeTransaction::LayerProperties::decode):
Make it possible to remove the RemoteLayerBackingStore on layers
that went from drawing contents to not drawing contents.

* UIProcess/ios/ViewGestureControllerIOS.mm:
(WebKit::ViewGestureController::beginSwipeGesture):
(WebKit::ViewGestureController::removeSwipeSnapshot):
* UIProcess/mac/ViewGestureControllerMac.mm:
(WebKit::ViewGestureController::beginSwipeGesture):
(WebKit::ViewGestureController::removeSwipeSnapshot):
* UIProcess/mac/ViewSnapshotStore.mm:
(WebKit::ViewSnapshotStore::recordSnapshot):
Adopt the new names from WebCore.

* WebKit2.xcodeproj/project.pbxproj:
* WebProcess/WebPage/mac/PlatformCALayerRemote.cpp:
(WebKit::PlatformCALayerRemote::recursiveBuildTransaction):
Clear the backing store if the layer doesn't want to draw contents anymore.

(WebKit::PlatformCALayerRemote::ensureBackingStore):

* WebProcess/WebPage/mac/RemoteLayerTreeContext.h:
(WebKit::RemoteLayerTreeContext::backingStoreCollection):
* WebProcess/WebPage/mac/RemoteLayerTreeContext.mm:
(WebKit::RemoteLayerTreeContext::RemoteLayerTreeContext):
(WebKit::RemoteLayerTreeContext::backingStoreWasCreated):
(WebKit::RemoteLayerTreeContext::backingStoreWillBeDestroyed):
Keep a RemoteLayerBackingStoreCollection, which just keeps track
of all backing store (even unparented) created for layers in a
particular context. Forward backing store creation/deletion to it.
Remove a unused m_rootLayer member.

* WebProcess/WebPage/mac/RemoteLayerTreeDrawingArea.mm:
(WebKit::RemoteLayerTreeDrawingArea::flushLayers):
If any layers had backing store changes, start the timer
on the RemoteLayerBackingStoreCollection that will mark them
purgeable once they are no longer needed.

* Shared/mac/RemoteLayerBackingStoreCollection.mm:
* Shared/mac/RemoteLayerBackingStoreCollection.h:
Keep track of all backing store for our context.
When the timer fires, run through the backing store, finding
backing store that was last painted more than 1 second ago,
and mark them purgeable. If there was more recently-painted
backing store, let the timer fire again in another second;
otherwise, stop it.

* WebCore.exp.in:
* platform/graphics/cg/IOSurfacePool.cpp:
(WebCore::IOSurfacePool::takeSurface):
(WebCore::IOSurfacePool::markOlderSurfacesPurgeable):
(WebCore::IOSurfacePool::showPoolStatistics):
* platform/graphics/cocoa/IOSurface.h:
* platform/graphics/cocoa/IOSurface.mm:
(IOSurface::isVolatile):
(IOSurface::setIsVolatile):
Rename purgeability functions to match the system
and reduce confusion between purgeable memory
and volatile vs. non-volatile memory.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@166886 268f45cc-cd09-0410-ab3c-d52691b4dbfc