Move WebGraphicsContext3D related tests from web/tests/ to platform/graphics/gpu/

Move Canvas2DLayerBridgeTest.cpp, Canvas2DLayerManagerTest.cpp and
DrawingBufferTest.cpp.

BUG=353585

Review URL: https://codereview.chromium.org/203513002

git-svn-id: svn://svn.chromium.org/blink/trunk@169474 bbb929c8-8fbe-4397-9dbb-9b2b20218538