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 4e777c0fe325aaae71866b1bc6809b22913d5975..472287d1494224babfd97fd9085734ea9d57365d 100644
--- a/Source/core/core.gypi
+++ b/Source/core/core.gypi
@@ -110,6 +110,7 @@
             'events/PageTransitionEvent.idl',
             'events/PopStateEvent.idl',
             'events/ProgressEvent.idl',
+            'events/RelatedEvent.idl',
             'events/ResourceProgressEvent.idl',
             'events/SecurityPolicyViolationEvent.idl',
             'events/TextEvent.idl',
@@ -178,6 +179,7 @@
             'html/HTMLMarqueeElement.idl',
             'html/HTMLMediaElement.idl',
             'html/HTMLMenuElement.idl',
+            'html/HTMLMenuItemElement.idl',
             'html/HTMLMetaElement.idl',
             'html/HTMLMeterElement.idl',
             'html/HTMLModElement.idl',
@@ -509,6 +511,7 @@
             'events/PageTransitionEvent.idl',
             'events/PopStateEvent.idl',
             'events/ProgressEvent.idl',
+            'events/RelatedEvent.idl',
             'events/ResourceProgressEvent.idl',
             'events/SecurityPolicyViolationEvent.idl',
             'events/TextEvent.idl',
@@ -1410,6 +1413,8 @@
             'page/ContextMenuProvider.h',
             'page/CreateWindow.cpp',
             'page/CreateWindow.h',
+            'page/CustomContextMenuProvider.cpp',
+            'page/CustomContextMenuProvider.h',
             'page/DOMSelection.cpp',
             'page/DOMSelection.h',
             'page/DOMWindowPagePopup.cpp',
@@ -2256,6 +2261,8 @@
             'events/PopStateEvent.h',
             'events/ProgressEvent.cpp',
             'events/ProgressEvent.h',
+            'events/RelatedEvent.cpp',
+            'events/RelatedEvent.h',
             'events/ResourceProgressEvent.cpp',
             'events/ResourceProgressEvent.h',
             'events/ScopedEventQueue.cpp',
@@ -2378,6 +2385,8 @@
             'html/HTMLMediaSource.h',
             'html/HTMLMenuElement.cpp',
             'html/HTMLMenuElement.h',
+            'html/HTMLMenuItemElement.cpp',
+            'html/HTMLMenuItemElement.h',
             'html/HTMLMetaElement.h',
             'html/HTMLMeterElement.cpp',
             'html/HTMLMeterElement.h',
@@ -3436,6 +3445,7 @@
             'html/track/vtt/BufferedLineReaderTest.cpp',
             'html/track/vtt/VTTScannerTest.cpp',
             'loader/MixedContentCheckerTest.cpp',
+            'page/ContextMenuControllerTest.cpp',
             'page/NetworkStateNotifierTest.cpp',
             'rendering/RenderOverflowTest.cpp',
             'rendering/RenderTableCellTest.cpp',