Decouple cc::FakeWebGraphicsContext3D from blink::WGC3D

cc_unittests that do not need a real OpenGL context use a stubbed-out
context provided by cc::TestGLES2Interface that forwards to a cc::TestWebGraphicsContext3D
stub that is subclassed by various unit tests. This makes cc::TWGC3D no longer subclass
blink::WebGraphicsContext3D to cut down on the blink dependency. This will enable moving
the test code to derive from TestGLES2Interface more directly down the road to cut out
the intermediates.

One subtle consequence of this change is that now that no class in the blink namespace is
in the inheritance chain for the test context subclasses, the clang checker now requires
OVERRIDE annotations.

BUG=181120

Review URL: https://codereview.chromium.org/126973002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244152 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244152