Bug 1565927 - Default FrameCrashedEvent's isTopFrame to true if never set. r=nika

This is the safer assumption, since we normally skip setting isTopFrame
explicitly if no BrowsingContext exists (which is the case if, for example,
the crash is due to the fact that creating the top-level browser failed).

Differential Revision: https://phabricator.services.mozilla.com/D38119

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

dom/webidl/FrameCrashedEvent.webidl

@@ -21,5 +21,5 @@ interface FrameCrashedEvent : Event
 dictionary FrameCrashedEventInit : EventInit
 {
   unsigned long long browsingContextId = 0;
-  boolean isTopFrame = false;
+  boolean isTopFrame = true;
 };