Deprecate the allowfullscreen exemption for <video>

Source/core/frame/UseCounter.cpp

Index: Source/core/frame/UseCounter.cpp
diff --git a/Source/core/frame/UseCounter.cpp b/Source/core/frame/UseCounter.cpp
index 794e5246280616c7eac16ece4dcd6f35f3e28642..7e68504b5c512ffa69e56b6b2280692430112762 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -928,6 +928,9 @@ String UseCounter::deprecationMessage(Feature feature)
     case PrefixedMouseEventMovementY:
         return replacedBy("webkitMovementY", "movementY");
 
+    case VideoFullscreenAllowedExemption:
+        return "Entering fullscreen in an <iframe> with no allowfullscreen attribute is deprecated and will stop working in M46, around October 2015. Please use the allowfullscreen attribute.";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();