UseCounter: Introduce UseCounter::Observer for layout tests

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

Index: third_party/WebKit/Source/core/testing/Internals.h
diff --git a/third_party/WebKit/Source/core/testing/Internals.h b/third_party/WebKit/Source/core/testing/Internals.h
index 71751dd3725cc7a2fc7c96c4b5cc3fc633693c01..48849dc086a3c0564e2edce95846c989ee871e8c 100644
--- a/third_party/WebKit/Source/core/testing/Internals.h
+++ b/third_party/WebKit/Source/core/testing/Internals.h
@@ -493,6 +493,11 @@ class Internals final : public GarbageCollected<Internals>,
   bool isUseCounted(Document*, uint32_t feature);
   bool isCSSPropertyUseCounted(Document*, const String&);
 
+  // Observes changes on Document's UseCounter. Returns a promise that is
+  // resolved when |feature| is counted. When |feature| was already counted,
+  // it's immediately resolved.
+  ScriptPromise observeUseCounter(ScriptState*, Document*, uint32_t feature);
+
   // Used by the iterable<>.
   unsigned length() const { return 5; }
   int anonymousIndexedGetter(uint32_t index) const { return index * index; }