extensions: Speculative fix for crash on IO thread.

Make ExtensionFunctionDispatcher::DispatchOnIOThread()
return early if the extension that was passed to it doesn't
exist anymore.

BUG=406062

Review URL: https://codereview.chromium.org/489033003

Cr-Commit-Position: refs/heads/master@{#291618}

extensions/browser/extension_function_dispatcher.cc

@@ -231,6 +231,8 @@ void ExtensionFunctionDispatcher::DispatchOnIOThread(
     const ExtensionHostMsg_Request_Params& params) {
   const Extension* extension =
       extension_info_map->extensions().GetByID(params.extension_id);
+  if (!extension)
+    return;
 
   ExtensionFunction::ResponseCallback callback(
       base::Bind(&IOThreadResponseCallback, ipc_sender, routing_id,