UseCounter: Introduce UseCounter::Observer for layout tests

third_party/WebKit/Source/core/testing/Internals.idl

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  * Copyright (C) 2013 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 313 matching lines @@
     int visualViewportHeight();
     int visualViewportWidth();
     double visualViewportScrollX();
     double visualViewportScrollY();
 
     bool magnifyScaleAroundAnchor(float offset, float x, float y);
 
     boolean isUseCounted(Document document, unsigned long feature);
     boolean isCSSPropertyUseCounted(Document document, DOMString propertyName);
 
+    // Returns a promise that is resolved when |feature| is counted on
+    // |document|'s UseCounter. When |feature| was already counted, it's
+    // immediately resolved.
+    [CallWith=ScriptState] Promise<bool> observeUseCounter(Document document, unsigned long feature);
+
     readonly attribute unsigned long length;
     getter long (unsigned long index);
     iterable<long>;
 
     [Unscopable] readonly attribute DOMString unscopableAttribute;
     [Unscopable] DOMString unscopableMethod();
 
     ClientRectList focusRingRects(Element element);
     ClientRectList outlineRects(Element element);
     void setCapsLockState(boolean enabled);
@@ skipping 11 matching lines @@
     DOMString getProgrammaticScrollAnimationState(Node node);
 
     ClientRect visualRect(Node node);
 
     OriginTrialsTest originTrialsTest();
 
     void crash();
 
     void setIsLowEndDevice(boolean isLowEndDevice);
 };