Add UseCounter for FontFaceSet#ready()

In the latest editor's draft spec [1], FontFaceSet#ready has been
changed from a method to an attribute. We are likely to deprecate
ready() in favor of ready attribute. To prepare for a deprecation
process, this patch adds a UseCounter for FontFaceSet#ready().

[1] http://dev.w3.org/csswg/css-font-loading/#dom-fontfaceset-ready

BUG=380990

Review URL: https://codereview.chromium.org/339563006

git-svn-id: svn://svn.chromium.org/blink/trunk@176288 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/css/FontFaceSet.idl

@@ -43,7 +43,7 @@ enum FontFaceSetLoadStatus { "loading", "loaded" };
 
     [RaisesException] boolean check(DOMString font, optional DOMString text = null);
     [CallWith=ScriptState] Promise load(DOMString font, optional DOMString text = null);
-    [CallWith=ScriptState] Promise ready();
+    [MeasureAs=FontFaceSetReady, CallWith=ScriptState] Promise ready();
 
     [RaisesException] void add(FontFace fontFace);
     void clear();

third_party/WebKit/Source/core/frame/UseCounter.h

@@ -461,6 +461,7 @@ public:
         EditingAppleStyleSpanClass = 461,
         EditingAppleTabSpanClass = 462,
         HTMLImportsAsyncAttribute = 463,
+        FontFaceSetReady = 464,
         // Add new features immediately above this line. Don't change assigned
         // numbers of any item, and don't reuse removed slots.
         // Also, run update_use_counter_feature_enum.py in chromium/src/tools/metrics/histograms/