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 5b8c58f59f6e12fa711694e4b76c10a14c4234af..2c311276845f7ca87e26345f3e71c68f971dcb4e 100644
--- a/Source/core/core.gypi
+++ b/Source/core/core.gypi
@@ -106,6 +106,7 @@
             'events/PageTransitionEvent.idl',
             'events/PopStateEvent.idl',
             'events/ProgressEvent.idl',
+            'events/RelatedEvent.idl',
             'events/ResourceProgressEvent.idl',
             'events/SecurityPolicyViolationEvent.idl',
             'events/TextEvent.idl',
@@ -175,6 +176,7 @@
             'html/HTMLMarqueeElement.idl',
             'html/HTMLMediaElement.idl',
             'html/HTMLMenuElement.idl',
+            'html/HTMLMenuitemElement.idl',
             'html/HTMLMetaElement.idl',
             'html/HTMLMeterElement.idl',
             'html/HTMLModElement.idl',
@@ -500,6 +502,7 @@
             'events/PageTransitionEvent.idl',
             'events/PopStateEvent.idl',
             'events/ProgressEvent.idl',
+            'events/RelatedEvent.idl',
             'events/ResourceProgressEvent.idl',
             'events/SecurityPolicyViolationEvent.idl',
             'events/TextEvent.idl',
@@ -2251,6 +2254,8 @@
             'events/PopStateEvent.h',
             'events/ProgressEvent.cpp',
             'events/ProgressEvent.h',
+            'events/RelatedEvent.cpp',
+            'events/RelatedEvent.h',
             'events/ResourceProgressEvent.cpp',
             'events/ResourceProgressEvent.h',
             'events/ScopedEventQueue.cpp',
@@ -2279,6 +2284,8 @@
         'webcore_html_files': [
             'html/ClassList.cpp',
             'html/ClassList.h',
+            'html/CustomContextMenuProvider.cpp',
+            'html/CustomContextMenuProvider.h',
             'html/DOMFormData.cpp',
             'html/DOMFormData.h',
             'html/DocumentNameCollection.cpp',
@@ -2373,6 +2380,8 @@
             'html/HTMLMediaSource.h',
             'html/HTMLMenuElement.cpp',
             'html/HTMLMenuElement.h',
+            'html/HTMLMenuitemElement.cpp',
+            'html/HTMLMenuitemElement.h',
             'html/HTMLMetaElement.h',
             'html/HTMLMeterElement.cpp',
             'html/HTMLMeterElement.h',