Fixes the following build failure for iOS:

    In file included from Source/WebCore/accessibility/AccessibilityAllInOne.cpp:28:
    In file included from Source/WebCore/accessibility/AXObjectCache.cpp:42:
    In file included from Source/WebCore/accessibility/AccessibilityMediaControls.h:36:
    In file included from Source/WebCore/html/shadow/MediaControlElements.h:34:
    In file included from Source/WebCore/html/shadow/MediaControlElementTypes.h:37:
    Source/WebCore/html/HTMLMediaElement.h:324:23: error: unknown type name 'PluginCreationOption'
        void updateWidget(PluginCreationOption);
                          ^

* html/HTMLMediaElement.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@154559 268f45cc-cd09-0410-ab3c-d52691b4dbfc