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 b87d5798d4dd3cb427cc44531b8df78666592b68..33d500d86b435a949ae27bc688e37b42a1a81e79 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',
@@ -497,6 +499,7 @@
             'events/PageTransitionEvent.idl',
             'events/PopStateEvent.idl',
             'events/ProgressEvent.idl',
+            'events/RelatedEvent.idl',
             'events/ResourceProgressEvent.idl',
             'events/SecurityPolicyViolationEvent.idl',
             'events/TextEvent.idl',
@@ -2239,6 +2242,8 @@
             'events/PopStateEvent.h',
             'events/ProgressEvent.cpp',
             'events/ProgressEvent.h',
+            'events/RelatedEvent.cpp',
+            'events/RelatedEvent.h',
             'events/ResourceProgressEvent.cpp',
             'events/ResourceProgressEvent.h',
             'events/ScopedEventQueue.cpp',
@@ -2267,6 +2272,8 @@
         'webcore_html_files': [
             'html/ClassList.cpp',
             'html/ClassList.h',
+            'html/CustomContextMenuProvider.cpp',
+            'html/CustomContextMenuProvider.h',
             'html/DOMFormData.cpp',
             'html/DOMFormData.h',
             'html/FormAssociatedElement.cpp',
@@ -2359,6 +2366,10 @@
             'html/HTMLMediaSource.h',
             'html/HTMLMenuElement.cpp',
             'html/HTMLMenuElement.h',
+            'html/HTMLMenuElementEventListener.cpp',
+            'html/HTMLMenuElementEventListener.h',
+            'html/HTMLMenuitemElement.cpp',
+            'html/HTMLMenuitemElement.h',
             'html/HTMLMetaElement.h',
             'html/HTMLMeterElement.cpp',
             'html/HTMLMeterElement.h',