Bug 1463919 - Check AutoplayPolicy before activating attribute based autoplay. r=jya

When autoplay is requested by setting the "autoplay" attribute, we should
check whether autoplay is allowed in HTMLMediaElement::CheckAutoplayDataReady()
and if not we should prompt for user consent.

This ensures that <video ... autoplay/> will prompt for consent when used on
a page without a pre-existing allow/block permission.


MozReview-Commit-ID: 77pJR2Ybn2i

dom/html/HTMLMediaElement.cpp

@@ -6180,10 +6180,6 @@ HTMLMediaElement::CanActivateAutoplay()
   // download is controlled by the script and there is no way to evaluate
   // MediaDecoder::CanPlayThrough().
 
-  if (!AutoplayPolicy::IsMediaElementAllowedToPlay(WrapNotNull(this))) {
-    return false;
-  }
-
   if (!HasAttr(kNameSpaceID_None, nsGkAtoms::autoplay)) {
     return false;
   }
@@ -6235,6 +6231,16 @@ HTMLMediaElement::CheckAutoplayDataReady()
     return;
   }
 
+  switch (AutoplayPolicy::IsAllowedToPlay(*this)) {
+    case Authorization::Blocked:
+      return;
+    case Authorization::Prompt:
+      EnsureAutoplayRequested(false);
+      return;
+    case Authorization::Allowed:
+      break;
+  }
+
   mPaused = false;
   // We changed mPaused which can affect AddRemoveSelfReference
   AddRemoveSelfReference();