Bug 967228 - Avoid exception in DOMIdentity.jsm (r=MattN)

dom/identity/DOMIdentity.jsm

@@ -221,6 +221,10 @@ this.DOMIdentity = {
     return this._serviceContexts.get(this._mmContexts.get(targetMM));
   },
 
+  hasContextForMM: function(targetMM) {
+    return this._mmContexts.has(targetMM);
+  },
+
   /*
    * Delete the RPWatchContext object for a given message manager.  Removes the
    * mapping both from _serviceContexts and _mmContexts.
@@ -344,6 +348,10 @@ this.DOMIdentity = {
   },
 
   _childProcessShutdown: function DOMIdentity__childProcessShutdown(targetMM) {
+    if (!this.hasContextForMM(targetMM)) {
+      return;
+    }
+
     this.getContextForMM(targetMM).RP.childProcessShutdown(targetMM);
     this.deleteContextForMM(targetMM);