Add UseCounter to see how often the "allowfullscreen" attribute is modified.

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

Index: third_party/WebKit/Source/core/frame/UseCounter.h
diff --git a/third_party/WebKit/Source/core/frame/UseCounter.h b/third_party/WebKit/Source/core/frame/UseCounter.h
index b3b48d2219d8a48391300c7d0ead53b370bd0760..7d2b9f1bae4dbefaf4ce9c1dec430221be4b90ab 100644
--- a/third_party/WebKit/Source/core/frame/UseCounter.h
+++ b/third_party/WebKit/Source/core/frame/UseCounter.h
@@ -1455,6 +1455,7 @@ class CORE_EXPORT UseCounter {
     CSSValueOnDemand = 1802,
     ServiceWorkerNavigationPreload = 1803,
     FullscreenRequestWithPendingElement = 1804,
+    HTMLIFrameElementAllowfullscreenAttributeSetAfterContentLoad = 1805,
 
     // Add new features immediately above this line. Don't change assigned
     // numbers of any item, and don't reuse removed slots.