Move GraphicsContextTest to platform/ and fix the test environment.

Source/platform/blink_platform.gypi

 {
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.cpp',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
       'Clock.h',
       'ColorChooser.cpp',
@@ skipping 876 matching lines @@
       'animation/UnitBezierTest.cpp',
       'clipboard/ClipboardUtilitiesTest.cpp',
       'fonts/FontCacheTest.cpp',
       'fonts/FontDescriptionTest.cpp',
       'fonts/FontTest.cpp',
       'fonts/android/FontCacheAndroidTest.cpp',
       'geometry/FloatPolygonTest.cpp',
       'geometry/FloatRoundedRectTest.cpp',
       'geometry/RegionTest.cpp',
       'geometry/RoundedRectTest.cpp',
+      'graphics/GraphicsContextTest.cpp',
       'graphics/ThreadSafeDataTransportTest.cpp',
       'graphics/test/MockDiscardablePixelRef.h',
       'image-decoders/ImageDecoderTest.cpp',
       'testing/ArenaTestHelpers.h',
       'testing/TreeTestHelpers.cpp',
       'testing/TreeTestHelpers.h',
       'text/BidiResolverTest.cpp',
       'text/DateTimeFormatTest.cpp',
       'text/SegmentedStringTest.cpp',
       'text/UnicodeUtilitiesTest.cpp',
@@ skipping 13 matching lines @@
       'graphics/ImageDecodingStoreTest.cpp',
       'graphics/ImageFrameGeneratorTest.cpp',
       'graphics/gpu/DrawingBufferTest.cpp',
       'graphics/test/MockImageDecoder.h',
       'graphics/test/MockWebGraphicsContext3D.h',
       'image-decoders/gif/GIFImageDecoderTest.cpp',
       'image-decoders/jpeg/JPEGImageDecoderTest.cpp',
     ],
   },
 }