Remove dead file MockDiscardablePixelRef.h from gypi file.

Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
@@ skipping 931 matching lines @@
       'geometry/GeometryTestHelpers.cpp',
       'geometry/LayoutRectOutsetsTest.cpp',
       'geometry/RegionTest.cpp',
       'graphics/GraphicsContextTest.cpp',
       'graphics/RecordingImageBufferSurfaceTest.cpp',
       'graphics/ThreadSafeDataTransportTest.cpp',
       'graphics/filters/FilterOperationsTest.cpp',
       'graphics/filters/ImageFilterBuilderTest.cpp',
       'graphics/gpu/DrawingBufferTest.cpp',
       'graphics/paint/DisplayItemTest.cpp',
-      'graphics/test/MockDiscardablePixelRef.h',
       'image-decoders/ImageDecoderTest.cpp',
       'mac/ScrollElasticityControllerTest.mm',
       'network/FormDataTest.cpp',
       'network/HTTPParsersTest.cpp',
       'network/ResourceRequestTest.cpp',
       'scheduler/CancellableTaskFactoryTest.cpp',
       'scheduler/SchedulerTest.cpp',
       'scroll/ScrollableAreaTest.cpp',
       'testing/ArenaTestHelpers.h',
       'testing/TreeTestHelpers.cpp',
@@ skipping 23 matching lines @@
       'graphics/ImageLayerChromiumTest.cpp',
       'graphics/test/MockImageDecoder.h',
       'graphics/test/MockWebGraphicsContext3D.h',
       'image-decoders/bmp/BMPImageDecoderTest.cpp',
       'image-decoders/gif/GIFImageDecoderTest.cpp',
       'image-decoders/jpeg/JPEGImageDecoderTest.cpp',
       'image-decoders/webp/WEBPImageDecoderTest.cpp',
     ],
   },
 }