Enable srcset support in HTMLImageElement

Source/core/core.gypi

 {
     'variables': {
         'core_idl_files': [
             'css/CSS.idl',
             'css/CSSCharsetRule.idl',
             'css/CSSFontFaceLoadEvent.idl',
             'css/CSSFontFaceRule.idl',
             'css/CSSHostRule.idl',
             'css/CSSImportRule.idl',
             'css/CSSKeyframeRule.idl',
@@ skipping 2477 matching lines @@
             'html/parser/HTMLParserThread.h',
             'html/parser/HTMLPreloadScanner.cpp',
             'html/parser/HTMLPreloadScanner.h',
             'html/parser/HTMLResourcePreloader.cpp',
             'html/parser/HTMLResourcePreloader.h',
             'html/parser/HTMLScriptRunner.cpp',
             'html/parser/HTMLScriptRunner.h',
             'html/parser/HTMLScriptRunnerHost.h',
             'html/parser/HTMLSourceTracker.cpp',
             'html/parser/HTMLSourceTracker.h',
+            'html/parser/HTMLSrcsetParser.cpp',
+            'html/parser/HTMLSrcsetParser.h',
             'html/parser/HTMLStackItem.h',
             'html/parser/HTMLToken.h',
             'html/parser/HTMLTokenizer.cpp',
             'html/parser/HTMLTokenizer.h',
             'html/parser/HTMLTreeBuilder.cpp',
             'html/parser/HTMLTreeBuilder.h',
             'html/parser/HTMLTreeBuilderSimulator.cpp',
             'html/parser/HTMLTreeBuilderSimulator.h',
             'html/parser/HTMLViewSourceParser.cpp',
             'html/parser/HTMLViewSourceParser.h',
@@ skipping 1382 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++"'
               }],
          ],
     }
 }