Add a UseCounter for the webkitallowfullscreen attribute

We support the unprefixed allowfullscreen attribute, so it would be nice
to also reflect it in HTMLIFrameElement.allowFullscreen. However, that's
tricky while there are two backing attributes, so first measure if the
prefixed attribute can be removed yet.

BUG=none

Review URL: https://codereview.chromium.org/212433004

git-svn-id: svn://svn.chromium.org/blink/trunk@170124 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/dom/FullscreenElementStack.cpp

@@ -34,6 +34,7 @@
 #include "core/frame/FrameHost.h"
 #include "core/frame/LocalFrame.h"
 #include "core/frame/Settings.h"
+#include "core/frame/UseCounter.h"
 #include "core/html/HTMLFrameOwnerElement.h"
 #include "core/html/HTMLMediaElement.h"
 #include "core/page/Chrome.h"
@@ -45,13 +46,18 @@ namespace WebCore {
 
 using namespace HTMLNames;
 
-static bool isAttributeOnAllOwners(const WebCore::QualifiedName& attribute, const WebCore::QualifiedName& prefixedAttribute, const HTMLFrameOwnerElement* owner)
+static bool fullscreenIsAllowedForAllOwners(const Document& document)
 {
+    const HTMLFrameOwnerElement* owner = document.ownerElement();
     if (!owner)
         return true;
     do {
-        if (!(owner->hasAttribute(attribute) || owner->hasAttribute(prefixedAttribute)))
-            return false;
+        if (!owner->hasAttribute(allowfullscreenAttr)) {
+            if (owner->hasAttribute(webkitallowfullscreenAttr))
+                UseCounter::count(document, UseCounter::PrefixedAllowFullscreenAttribute);
+            else
+                return false;
+        }
     } while ((owner = owner->document().ownerElement()));
     return true;
 }
@@ -134,7 +140,7 @@ void FullscreenElementStack::documentWasDisposed()
 bool FullscreenElementStack::fullScreenIsAllowedForElement(Element* element) const
 {
     ASSERT(element);
-    return isAttributeOnAllOwners(allowfullscreenAttr, webkitallowfullscreenAttr, element->document().ownerElement());
+    return fullscreenIsAllowedForAllOwners(element->document());
 }
 
 void FullscreenElementStack::requestFullScreenForElement(Element* element, unsigned short flags, FullScreenCheckType checkType)
@@ -359,8 +365,7 @@ bool FullscreenElementStack::webkitFullscreenEnabled(Document& document)
     // browsing context's documents have their fullscreen enabled flag set, or false otherwise.
 
     // Top-level browsing contexts are implied to have their allowFullScreen attribute set.
-    return isAttributeOnAllOwners(allowfullscreenAttr, webkitallowfullscreenAttr, document.ownerElement());
-
+    return fullscreenIsAllowedForAllOwners(document);
 }
 
 void FullscreenElementStack::webkitWillEnterFullScreenForElement(Element* element)

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

@@ -329,6 +329,7 @@ public:
         NamedNodeMapRemoveNamedItemNS = 312,
         OpenWebDatabaseInWorker = 313,
         OpenWebDatabaseSyncInWorker = 314,
+        PrefixedAllowFullscreenAttribute = 315,
         // Add new features immediately above this line. Don't change assigned
         // numbers of any item, and don't reuse removed slots.
         NumberOfFeatures, // This enum value must be last.