Bug 1168180 - Mark nsPresContext::mShell as MOZ_NON_OWNING_REF. r=dbaron

--HG--
extra : rebase_source : d2bd620caa680978c30f0797f07169625e97c65c

layout/base/nsPresContext.h

@@ -1217,7 +1217,9 @@ protected:
   // please make the ownership explicit (pinkerton, scc).
 
   nsPresContextType     mType;
-  nsIPresShell*         mShell;         // [WEAK]
+  // the nsPresShell owns a strong reference to the nsPresContext, and is responsible
+  // for nulling this pointer before it is destroyed
+  nsIPresShell* MOZ_NON_OWNING_REF mShell;         // [WEAK]
   nsCOMPtr<nsIDocument> mDocument;
   nsRefPtr<nsDeviceContext> mDeviceContext; // [STRONG] could be weak, but
                                             // better safe than sorry.