UseCounter: Introduce UseCounter::Observer for layout tests

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

Index: third_party/WebKit/Source/core/testing/Internals.idl
diff --git a/third_party/WebKit/Source/core/testing/Internals.idl b/third_party/WebKit/Source/core/testing/Internals.idl
index 372f2abe6bf0f71d9b36ca2a89256ddb53d9b768..33563cf67ac0e991600baf1ea68af569831b8232 100644
--- a/third_party/WebKit/Source/core/testing/Internals.idl
+++ b/third_party/WebKit/Source/core/testing/Internals.idl
@@ -331,6 +331,11 @@
     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>;