Bug 899761 - Fix a debug-build shutdown crash in ContentParent. r=khuey

Sometimes a ContentParent object gets destroyed after XPCOM shutdown has
shut down the cycle collector.  If the ContentParent destructor releases
a cycle collected object (e.g. mMessageManager), this causes the cycle
collector to assert.

dom/ipc/ContentParent.cpp

@@ -849,6 +849,13 @@ ContentParent::ShutDownProcess(bool aCloseWithError)
   // NB: must MarkAsDead() here so that this isn't accidentally
   // returned from Get*() while in the midst of shutdown.
   MarkAsDead();
+
+  // A ContentParent object might not get freed until after XPCOM shutdown has
+  // shut down the cycle collector.  But by then it's too late to release any
+  // CC'ed objects, so we need to null them out here, while we still can.  See
+  // bug 899761.
+  mMemoryReporters.Clear();
+  mMessageManager = nullptr;
 }
 
 void

dom/ipc/ContentParent.h

@@ -430,6 +430,10 @@ private:
 
     virtual void ProcessingError(Result what) MOZ_OVERRIDE;
 
+    // If you add strong pointers to cycle collected objects here, be sure to
+    // release these objects in ShutDownProcess.  See the comment there for more
+    // details.
+
     GeckoChildProcessHost* mSubprocess;
     base::ChildPrivileges mOSPrivileges;