Add a mechanism to test for the compositing tree mutated during painting

Add a mechanism to test for the compositing tree mutated during painting
https://bugs.webkit.org/show_bug.cgi?id=68738

Patch by James Robinson <jamesr@chromium.org> on 2011-09-27
Reviewed by Adam Barth.

Source/WebCore:

Sets a static bool during GraphicsLayer::paintGraphicsLayerContents and ASSERT()s that we never create or
destroy a GraphicsLayer inside this function. Painting should never mutate the GraphicsLayer tree.

Test: compositing/video/video-with-invalid-source.html

* platform/graphics/GraphicsLayer.cpp:
(WebCore::GraphicsLayer::GraphicsLayer):
(WebCore::GraphicsLayer::~GraphicsLayer):
(WebCore::GraphicsLayer::paintGraphicsLayerContents):

LayoutTests:

Adds a test that caused compositing to be disabled during painting before r95863 due to a video load failing.

* compositing/video/video-with-invalid-source-expected.txt: Added.
* compositing/video/video-with-invalid-source.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@96160 268f45cc-cd09-0410-ab3c-d52691b4dbfc