Move GraphicsContextTest to platform/ and fix the test environment.

Source/web/web.gypi

 {
   'variables': {
     'web_files': [
       'AssertMatchingEnums.cpp',
       'AssociatedURLLoader.cpp',
       'AssociatedURLLoader.h',
       'BackForwardClientImpl.cpp',
       'BackForwardClientImpl.h',
       'ChromeClientImpl.cpp',
       'ChromeClientImpl.h',
@@ skipping 252 matching lines @@
       'tests/ChromeClientImplTest.cpp',
       'tests/CompositedLayerMappingTest.cpp',
       'tests/CustomEventTest.cpp',
       'tests/DragImageTest.cpp',
       'tests/FakeWebPlugin.cpp',
       'tests/FakeWebPlugin.h',
       'tests/FilterOperationsTest.cpp',
       'tests/FrameLoaderClientImplTest.cpp',
       'tests/FrameTestHelpers.cpp',
       'tests/FrameTestHelpers.h',
-      'tests/GraphicsContextTest.cpp',
       'tests/GraphicsLayerTest.cpp',
       'tests/ImageFilterBuilderTest.cpp',
       'tests/ImageLayerChromiumTest.cpp',
       'tests/KeyboardTest.cpp',
       'tests/LinkHighlightTest.cpp',
       'tests/ListenerLeakTest.cpp',
       'tests/MemoryInfo.cpp',
       'tests/MHTMLTest.cpp',
       'tests/OpaqueRectTrackingContentLayerDelegateTest.cpp',
       'tests/OpenTypeVerticalDataTest.cpp',
@@ skipping 64 matching lines @@
         {
           'web_unittest_files': [
             'tests/KeyCodeConversionTestGtk.cpp',
             'tests/WebInputEventFactoryTestGtk.cpp',
           ],
         }
       ],
     ],
   },
 }