Add support to populate custom context menu items.

Source/core/core.gypi

 {
     'includes': [
       'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/Animation.idl',
             'animation/AnimationEffect.idl',
             'animation/AnimationPlayer.idl',
@@ skipping 1407 matching lines @@
             'loader/appcache/ApplicationCacheHost.cpp',
             'loader/appcache/ApplicationCacheHost.h',
             'page/AutoscrollController.cpp',
             'page/AutoscrollController.h',
             'page/BackForwardClient.h',
             'page/Chrome.cpp',
             'page/ContextMenuController.cpp',
             'page/ContextMenuProvider.h',
             'page/CreateWindow.cpp',
             'page/CreateWindow.h',
+            'page/CustomContextMenuProvider.cpp',
+            'page/CustomContextMenuProvider.h',
             'page/DOMWindowPagePopup.cpp',
             'page/DOMWindowPagePopup.h',
             'page/DragController.cpp',
             'page/DragData.cpp',
             'page/EventHandler.cpp',
             'page/EventSource.cpp',
             'page/EventSource.h',
             'page/EventWithHitTestResults.h',
             'page/FocusController.cpp',
             'page/FocusType.h',
@@ skipping 2028 matching lines @@
             'rendering/shapes/BoxShapeTest.cpp',
             'rendering/style/OutlineValueTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'streams/ReadableStreamTest.cpp',
             'testing/UnitTestHelpers.cpp',
             'testing/UnitTestHelpers.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }