Restore the HTMLVideoElement-specific prefixed fullscreen API behind a flag

Source/core/html/HTMLMediaElement.cpp

Index: Source/core/html/HTMLMediaElement.cpp
diff --git a/Source/core/html/HTMLMediaElement.cpp b/Source/core/html/HTMLMediaElement.cpp
index f5fb8f9311ddbe67a339b11b7c983bc1d9e04ec9..b027b138187af813fe690c5b5a2af81613f3c960 100644
--- a/Source/core/html/HTMLMediaElement.cpp
+++ b/Source/core/html/HTMLMediaElement.cpp
@@ -3189,6 +3189,14 @@ void HTMLMediaElement::enterFullscreen()
         FullscreenElementStack::from(document()).requestFullScreenForElement(this, 0, FullscreenElementStack::ExemptIFrameAllowFullScreenRequirement);
 }
 
+void HTMLMediaElement::exitFullscreen()
+{
+    WTF_LOG(Media, "HTMLMediaElement::exitFullscreen");
+
+    if (document().settings() && document().settings()->fullScreenEnabled() && isFullscreen())
+        FullscreenElementStack::from(document()).webkitCancelFullScreen();
+}
+
 void HTMLMediaElement::didBecomeFullscreenElement()
 {
     if (hasMediaControls())