[chromium] Clean up GraphicsContext3D initialization paths

[chromium] Clean up GraphicsContext3D initialization paths
https://bugs.webkit.org/show_bug.cgi?id=79321

Reviewed by Kenneth Russell.

This simplifies the GraphicsContext3D initialization paths down to two simple codepaths, one for offscreen
contexts initialized from WebCore and one for onscreen (compositor) contexts initialized by WebViewImpl or
WebLayerTreeViewImpl.

Offscreen initialization path:
1) WebCore code calls WebCore::GraphicsContext3D::create(), implemented in GraphicsContext3DChromium.cpp
2) GraphicsContext3D::create() instantiates a WebGraphicsContext3D via the static WebKitPlatformSupport interface
3) GraphicsContext3DPrivate::createGraphicsContextFromWebContext() wraps the WebGraphicsContext3D in a
GraphicsContext3D's m_private pointer.

Onscreen initialization path:
1) WebViewImpl or WebLayerTreeViewImpl request an onscreen WebGraphicsContext3D from either their WebViewClient
or WebLayerTreeViewClient, respectively
2) GraphicsContext3DPrivate::createGraphicsContextFromWebContext() wraps the WebGraphicsContext3D in a
GraphicsContext3D's m_private pointer.

There are no other initialization paths. Specifically, we do not support instantiating onscreen contexts from
within WebCore.

* src/GraphicsContext3DChromium.cpp:
(WebCore::GraphicsContext3DPrivate::GraphicsContext3DPrivate):
(WebCore::GraphicsContext3DPrivate::createGraphicsContextFromWebContext):
(WebCore):
(WebCore::GraphicsContext3DPrivate::platformTexture):
(WebCore::GraphicsContext3D::create):
* src/GraphicsContext3DPrivate.h:
(GraphicsContext3DPrivate):
* src/WebLayerTreeViewImpl.cpp:
(WebKit::WebLayerTreeViewImpl::createLayerTreeHostContext3D):
* src/WebViewImpl.cpp:
(std::getCompositorContextAttributes):
(WebKit::WebViewImpl::createCompositorGraphicsContext3D):
(WebKit):
(WebKit::WebViewImpl::createLayerTreeHostContext3D):
(WebKit::WebViewImpl::graphicsContext3D):
* src/WebViewImpl.h:
(WebViewImpl):
* tests/CCLayerTreeHostImplTest.cpp:
(WebKit::CCLayerTreeHostImplTest::createContext):
(CCLayerTreeHostImplTest):
(WebKit::TEST_F):
* tests/CCLayerTreeHostTest.cpp:
(WTF::MockLayerTreeHostClient::createLayerTreeHostContext3D):
* tests/Canvas2DLayerChromiumTest.cpp:
(WebCore::Canvas2DLayerChromiumTest::fullLifecycleTest):
* tests/CompositorFakeGraphicsContext3D.h:
(WebCore::createCompositorMockGraphicsContext3D):
* tests/FakeGraphicsContext3DTest.cpp:
(TEST):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108706 268f45cc-cd09-0410-ab3c-d52691b4dbfc