Implement contextmenu attribute with basic support of <menu>

Source/core/core.gypi

Index: Source/core/core.gypi
diff --git a/Source/core/core.gypi b/Source/core/core.gypi
index 8d7aaf45b2560df402311d4a0080fe30617a0f85..c8499acc4525a18f0e353cfa4d3fffffc8725cef 100644
--- a/Source/core/core.gypi
+++ b/Source/core/core.gypi
@@ -104,6 +104,7 @@
             'events/PageTransitionEvent.idl',
             'events/PopStateEvent.idl',
             'events/ProgressEvent.idl',
+            'events/RelatedEvent.idl',
             'events/ResourceProgressEvent.idl',
             'events/SecurityPolicyViolationEvent.idl',
             'events/TextEvent.idl',
@@ -173,6 +174,7 @@
             'html/HTMLMarqueeElement.idl',
             'html/HTMLMediaElement.idl',
             'html/HTMLMenuElement.idl',
+            'html/HTMLMenuitemElement.idl',
             'html/HTMLMetaElement.idl',
             'html/HTMLMeterElement.idl',
             'html/HTMLModElement.idl',
@@ -2175,6 +2177,8 @@
             'events/PopStateEvent.h',
             'events/ProgressEvent.cpp',
             'events/ProgressEvent.h',
+            'events/RelatedEvent.cpp',
+            'events/RelatedEvent.h',
             'events/ResourceProgressEvent.cpp',
             'events/ResourceProgressEvent.h',
             'events/ScopedEventQueue.cpp',
@@ -2293,6 +2297,8 @@
             'html/HTMLMediaElement.cpp',
             'html/HTMLMenuElement.cpp',
             'html/HTMLMenuElement.h',
+            'html/HTMLMenuitemElement.cpp',
+            'html/HTMLMenuitemElement.h',
             'html/HTMLMetaElement.h',
             'html/HTMLMeterElement.cpp',
             'html/HTMLMeterElement.h',