Reland again "Decode GIF image frames on demand".

Source/core/core.gypi

 {
     'variables': {
         'core_idl_files': [
             'css/CSS.idl',
             'css/CSSCharsetRule.idl',
             'css/CSSFilterRule.idl',
             'css/CSSFilterValue.idl',
             'css/CSSFontFaceLoadEvent.idl',
             'css/CSSFontFaceRule.idl',
             'css/CSSHostRule.idl',
@@ skipping 3642 matching lines @@
         ],
         'core_unittest_files': [
             'animation/DocumentTimelineTest.cpp',
             'animation/PlayerTest.cpp',
             'animation/TimedItemCalculationsTest.cpp',
             'animation/TimedItemTest.cpp',
             'platform/DecimalTest.cpp',
             'platform/SharedBufferTest.cpp',
             'platform/chromium/ClipboardChromiumTest.cpp',
             'platform/graphics/chromium/AnimationTranslationUtilTest.cpp',
+            'platform/image-decoders/ImageDecoderTest.cpp',
+            'platform/image-decoders/gif/GIFImageDecoderTest.cpp',
             'platform/text/DateTimeFormatTest.cpp',
             'tests/CachedImageTest.cpp',
             'tests/HeapGraphSerializerTest.cpp',
             'tests/LayoutUnit.cpp',
             'tests/LinkRelAttribute.cpp',
         ],
         'scripts_for_in_files': [
             'scripts/in_file.py',
             'scripts/in_generator.py',
             'scripts/license.py',
@@ skipping 17 matching lines @@
                 'bison_exe': 'bison',
                 # We specify a preprocess so it happens locally and won't get
                 # distributed to goma.
                 # FIXME: /usr/bin/gcc won't exist on OSX forever. We want to
                 # use /usr/bin/clang once we require Xcode 4.x.
                 'preprocessor': '--preprocessor "/usr/bin/gcc -E -P -x c++"'
               }],
          ],
     }
 }