Implement contextmenu attribute with basic support of <menu>

Source/web/web.gypi

 {
   'variables': {
     'web_files': [
       'AssertMatchingEnums.cpp',
       'AssociatedURLLoader.cpp',
       'AssociatedURLLoader.h',
       'BackForwardClientImpl.cpp',
       'BackForwardClientImpl.h',
       'ChromeClientImpl.cpp',
       'ChromeClientImpl.h',
@@ skipping 246 matching lines @@
       'painting/ContinuousPainter.cpp',
       'painting/ContinuousPainter.h',
       'painting/PaintAggregator.cpp',
       'painting/PaintAggregator.h',
       'win/WebFontRendering.cpp',
     ],
     'web_unittest_files': [
       'tests/AssociatedURLLoaderTest.cpp',
       'tests/ChromeClientImplTest.cpp',
       'tests/CompositedLayerMappingTest.cpp',
+      'tests/ContextMenuTest.cpp',
       'tests/CustomEventTest.cpp',
       'tests/FakeWebPlugin.cpp',
       'tests/FakeWebPlugin.h',
       'tests/FrameLoaderClientImplTest.cpp',
       'tests/FrameTestHelpers.cpp',
       'tests/FrameTestHelpers.h',
       'tests/KeyboardTest.cpp',
       'tests/LinkHighlightTest.cpp',
       'tests/ListenerLeakTest.cpp',
       'tests/MemoryInfo.cpp',
@@ skipping 59 matching lines @@
       ['os_posix==1 and OS!="mac"',
         {
           'web_unittest_files': [
             'tests/LocaleICUTest.cpp',
           ],
         }
       ],
     ],
   },
 }