Add a unit test for WebCore::GlyphBuffer.

Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
@@ skipping 877 matching lines @@
       'PurgeableVectorTest.cpp',
       'SharedBufferTest.cpp',
       'TestingPlatformSupport.cpp',
       'TestingPlatformSupport.h',
       'animation/TimingFunctionTest.cpp',
       'animation/UnitBezierTest.cpp',
       'clipboard/ClipboardUtilitiesTest.cpp',
       'fonts/FontCacheTest.cpp',
       'fonts/FontDescriptionTest.cpp',
       'fonts/FontTest.cpp',
+      'fonts/GlyphBufferTest.cpp',
       'fonts/GlyphPageTreeNodeTest.cpp',
       'fonts/android/FontCacheAndroidTest.cpp',
       'geometry/FloatBoxTest.cpp',
       'geometry/FloatBoxTestHelpers.cpp',
       'geometry/FloatPolygonTest.cpp',
       'geometry/FloatRoundedRectTest.cpp',
       'geometry/RegionTest.cpp',
       'geometry/RoundedRectTest.cpp',
       'graphics/GraphicsContextTest.cpp',
       'graphics/ThreadSafeDataTransportTest.cpp',
@@ skipping 30 matching lines @@
       'graphics/ImageLayerChromiumTest.cpp',
       'graphics/OpaqueRectTrackingContentLayerDelegateTest.cpp',
       'graphics/test/MockImageDecoder.h',
       'graphics/test/MockWebGraphicsContext3D.h',
       'image-decoders/gif/GIFImageDecoderTest.cpp',
       'image-decoders/jpeg/JPEGImageDecoderTest.cpp',
       'image-decoders/webp/WEBPImageDecoderTest.cpp',
     ],
   },
 }