Switch FontFaceSet#ready from a method to an attribute

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 f9f078aaac2cb558e9d78bd29c362731b60767f4..23246ff6914685d838031ab566463fa968b97d36 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -794,9 +794,6 @@ String UseCounter::deprecationMessage(Feature feature)
     case XMLHttpRequestSynchronousInNonWorkerOutsideBeforeUnload:
         return "Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check http://xhr.spec.whatwg.org/.";
 
-    case FontFaceSetReady:
-        return "document.fonts.ready() method is going to be replaced with document.fonts.ready attribute in future releases. Please be prepared. For more help, check https://code.google.com/p/chromium/issues/detail?id=392077#c3 .";
-
     case DOMImplementationHasFeatureReturnFalse:
         return "'DOMImplementation.hasFeature()' returning false is deprecated. Please do not use it, as per DOM it should always return true (https://dom.spec.whatwg.org/#dom-domimplementation-hasfeature).";