[iOS][wk2] Ensure that layers are marked volatile before allowing the process to...

[iOS][wk2] Ensure that layers are marked volatile before allowing the process to suspend
https://bugs.webkit.org/show_bug.cgi?id=134004
<rdar://problem/17186342>

Reviewed by Simon Fraser.

WebKit tries to make layers volatile when unparented, but sometimes isn't given
a chance to do so before the process gets suspended, so we end up with lots of
non-volatile surfaces that should really be volatile.

* Shared/mac/RemoteLayerBackingStoreCollection.h:
* Shared/mac/RemoteLayerBackingStoreCollection.mm:
(WebKit::RemoteLayerBackingStoreCollection::markBackingStoreVolatileImmediately):
(WebKit::RemoteLayerBackingStoreCollection::markAllBackingStoreVolatileImmediatelyIfPossible):
Add markAllBackingStoreVolatileImmediatelyIfPossible, which tries to mark *all*
buffers of *all* backing store, (live and unreachable), (front, back, and secondary),
volatile right away. It returns false if any buffer isn't marked volatile (because it was in-use).

* UIProcess/ios/ProcessThrottler.h:
* UIProcess/ios/ProcessThrottler.mm:
(WebKit::ProcessThrottler::updateAssertion):
(WebKit::ProcessThrottler::processReadyToSuspend):
(WebKit::ProcessThrottler::didCancelProcessSuspension):
* UIProcess/ios/WebProcessProxyIOS.mm:
(WebKit::WebProcessProxy::sendCancelProcessWillSuspend):
(WebKit::WebProcessProxy::didCancelProcessSuspension):
* UIProcess/WebProcessProxy.h:
* UIProcess/WebProcessProxy.messages.in:
* WebProcess/WebProcess.h:
* WebProcess/WebProcess.messages.in:
If the UI process is waiting for the Web process to confirm that it can suspend
and something happens (the view is reparented) that cancels the suspension, inform
the Web process that this happened, so that it can cancel any cleanup that might still be taking place.

* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::viewStateDidChange):
If a view goes in-window, dispatch the view state change immediately without delay,
to minimize the latency between coming in-window and being ready to go.

* WebProcess/WebPage/DrawingArea.h:
(WebKit::DrawingArea::markLayersVolatileImmediatelyIfPossible):
* WebProcess/WebPage/mac/RemoteLayerTreeDrawingArea.h:
* WebProcess/WebPage/mac/RemoteLayerTreeDrawingArea.mm:
(WebKit::RemoteLayerTreeDrawingArea::setRootCompositingLayer):
Schedule a flush when we change the root layer; otherwise, we can end up
detaching the root layer but changing nothing else, and never committing that change.

(WebKit::RemoteLayerTreeDrawingArea::markLayersVolatileImmediatelyIfPossible):

* WebProcess/WebProcess.cpp:
(WebKit::WebProcess::WebProcess):
(WebKit::WebProcess::processWillSuspend):
(WebKit::WebProcess::cancelProcessWillSuspend):
(WebKit::WebProcess::markAllLayersVolatileIfPossible):
(WebKit::WebProcess::processSuspensionCleanupTimerFired):
When the UI process is going to suspend the process, it sends us ProcessWillSuspend,
and defers the suspension until we send a ProcessReadyToSuspend back.
Delay ProcessReadyToSuspend until all layers in our process have been marked volatile.
We'll keep trying every 20ms until they're all volatile. For safety, the UI process will eventually
stop waiting for us, but the volatility change is usually applied successfully within the first
or second timer callback.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@170316 268f45cc-cd09-0410-ab3c-d52691b4dbfc