Bug 1592167 - part2 : using browsing content to check if the video document is in the top-level context. r=jya

Our policy is to allow video document to autoplay only when it's in the top-level window. However, `docshell::GetInProcessSameTypeRootTreeItem()` is not compatible with Fission so that we are not able to get the top-level document if we are in the remote process.

Therefore, we should use `BrowsingContext` instead, to check if the video document is in the top-level context.

Differential Revision: https://phabricator.services.mozilla.com/D51216

--HG--
extra : moz-landing-system : lando

dom/media/AutoplayPolicy.cpp

@@ -101,11 +101,19 @@ static bool IsWindowAllowedToPlay(nsPIDOMWindowInner* aWindow) {
     return true;
   }
 
-  if (!aWindow->GetExtantDoc()) {
+  Document* currentDoc = aWindow->GetExtantDoc();
+  if (!currentDoc) {
     return false;
   }
 
-  Document* approver = ApproverDocOf(*aWindow->GetExtantDoc());
+  bool isTopLevelContent = !aWindow->GetBrowsingContext()->GetParent();
+  if (currentDoc->MediaDocumentKind() == Document::MediaDocumentKind::Video &&
+      isTopLevelContent) {
+    AUTOPLAY_LOG("Allow top-level video document to autoplay.");
+    return true;
+  }
+
+  Document* approver = ApproverDocOf(*currentDoc);
   if (!approver) {
     return false;
   }
@@ -115,11 +123,6 @@ static bool IsWindowAllowedToPlay(nsPIDOMWindowInner* aWindow) {
     return true;
   }
 
-  if (approver->MediaDocumentKind() == Document::MediaDocumentKind::Video) {
-    AUTOPLAY_LOG("Allow video document to autoplay.");
-    return true;
-  }
-
   return false;
 }