LayerPool’d PlatformCALayerRemote/RemoteLayerBackingStore can have stale context...

LayerPool’d PlatformCALayerRemote/RemoteLayerBackingStore can have stale context pointers
https://bugs.webkit.org/show_bug.cgi?id=134050
<rdar://problem/17315114>

Reviewed by Simon Fraser.

* WebProcess/WebPage/mac/PlatformCALayerRemote.cpp:
(WebKit::PlatformCALayerRemote::recursiveBuildTransaction):
(WebKit::PlatformCALayerRemote::layerPool):
* WebProcess/WebPage/mac/PlatformCALayerRemote.h:
* WebProcess/WebPage/mac/RemoteLayerTreeContext.h:
(WebKit::RemoteLayerTreeContext::layerPool):
* WebProcess/WebPage/mac/RemoteLayerTreeContext.mm:
(WebKit::RemoteLayerTreeContext::buildTransaction):
Ensure that layers being committed are always in the context that is committing them.
Use a LayerPool per RemoteLayerTreeContext, to avoid moving layers between contexts.

* WebCore.exp.in:
* WebCore.xcodeproj/project.pbxproj:
* platform/cocoa/MemoryPressureHandlerCocoa.mm:
(WebCore::MemoryPressureHandler::platformReleaseMemory):
Drain all LayerPools upon memory pressure.

* platform/graphics/ca/LayerPool.cpp:
(WebCore::LayerPool::LayerPool):
(WebCore::LayerPool::~LayerPool):
(WebCore::LayerPool::allLayerPools):
(WebCore::LayerPool::sharedPool): Deleted.
* platform/graphics/ca/LayerPool.h:
* platform/graphics/ca/PlatformCALayer.cpp:
(WebCore::PlatformCALayer::createCompatibleLayerWithSize):
Add createCompatibleLayerWithSize, which optionally tries to take said layer from the LayerPool.

(WebCore::PlatformCALayer::moveToLayerPool):
Add moveToLayerPool, which moves the layer to the LayerPool.

(WebCore::PlatformCALayer::layerPool):
The LayerPool for each layer is obtained via PlatformCALayer::layerPool(), which subclasses can override for special behaviors.
The base class uses a global LayerPool.

* platform/graphics/ca/PlatformCALayer.h:
* platform/graphics/ca/TileController.cpp:
(WebCore::TileController::createTileLayer):
* platform/graphics/ca/TileGrid.cpp:
(WebCore::TileGrid::removeTiles):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@170179 268f45cc-cd09-0410-ab3c-d52691b4dbfc