Restore the HTMLVideoElement-specific prefixed fullscreen API behind a flag

Source/core/html/HTMLVideoElement.h

Index: Source/core/html/HTMLVideoElement.h
diff --git a/Source/core/html/HTMLVideoElement.h b/Source/core/html/HTMLVideoElement.h
index b4d92d1096090a850897b8e4478020889359f2cc..e14b4353fe1ed973b5084865ec55074133de3ef4 100644
--- a/Source/core/html/HTMLVideoElement.h
+++ b/Source/core/html/HTMLVideoElement.h
@@ -45,6 +45,12 @@ public:
     unsigned videoWidth() const;
     unsigned videoHeight() const;
 
+    // Fullscreen
+    void webkitEnterFullscreen(ExceptionState&);
+    void webkitExitFullscreen();
+    bool webkitSupportsFullscreen();
+    bool webkitDisplayingFullscreen();
+
     // Statistics
     unsigned webkitDecodedFrameCount() const;
     unsigned webkitDroppedFrameCount() const;
@@ -79,6 +85,7 @@ private:
     virtual bool isPresentationAttribute(const QualifiedName&) const OVERRIDE;
     virtual void collectStyleForPresentationAttribute(const QualifiedName&, const AtomicString&, MutableStylePropertySet*) OVERRIDE;
     virtual bool hasVideo() const OVERRIDE { return player() && player()->hasVideo(); }
+    bool supportsFullscreen() const;
     virtual bool isURLAttribute(const Attribute&) const OVERRIDE;
     virtual const AtomicString imageSourceURL() const OVERRIDE;