[Extensions] Handle null ScriptContexts in GetModuleSystem()

extensions/renderer/script_context_set.h

Index: extensions/renderer/script_context_set.h
diff --git a/extensions/renderer/script_context_set.h b/extensions/renderer/script_context_set.h
index 9f47d0ad95dedbec7000e86df79b72eba0e042c8..049d315e9ed10af5116cb3597a34158fc3b3eabe 100644
--- a/extensions/renderer/script_context_set.h
+++ b/extensions/renderer/script_context_set.h
@@ -75,6 +75,12 @@ class ScriptContextSet {
 
   // Returns the ScriptContext corresponding to the V8 context that created the
   // given |object|.
+  // Note: The provided |object| may belong to a v8::Context in another frame,
+  // as can happen when a parent frame uses an object of an embedded iframe.
+  // In this case, there may be no associated ScriptContext, since the child
+  // frame can be hosted in another process. Thus, callers of this need to
+  // null-check the result (and should also always check whether or not the
+  // context has access to the other context).
   static ScriptContext* GetContextByObject(const v8::Local<v8::Object>& object);
 
   // Synchronously runs |callback| with each ScriptContext that belongs to