Bug 1466926 - part1 : always allow extension background script to autoplay. r=cpearce

In present web extension design (both Firefox and Chrome), background script can autoplay.
We don't want to break this design, so we would always allow it to autoplay.

MozReview-Commit-ID: 9BfWgll7PNx

--HG--
extra : rebase_source : 7996daa06c31a84a2aea9008daa9bddfcde98c74

dom/base/nsDocument.cpp

@@ -12638,6 +12638,12 @@ nsIDocument::HasBeenUserGestureActivated()
   return mUserGestureActivated;
 }
 
+bool
+nsIDocument::IsExtensionPage() const
+{
+  return BasePrincipal::Cast(NodePrincipal())->AddonPolicy();
+}
+
 nsIDocument*
 nsIDocument::GetSameTypeParentDocument()
 {

dom/base/nsIDocument.h

@@ -3439,6 +3439,10 @@ public:
   // document in the document tree.
   bool HasBeenUserGestureActivated();
 
+  // This document is a WebExtension page, it might be a background page, a
+  // popup, a visible tab, a visible iframe ...e.t.c.
+  bool IsExtensionPage() const;
+
   bool HasScriptsBlockedBySandbox();
 
   bool InlineScriptAllowedByCSP();

dom/media/AutoplayPolicy.cpp

@@ -71,6 +71,11 @@ IsWindowAllowedToPlay(nsPIDOMWindowInner* aWindow)
     return true;
   }
 
+  if (approver->IsExtensionPage()) {
+    // Always allow extension page to autoplay.
+    return true;
+  }
+
   return false;
 }