Don't serialize extension user script injection results unless the extension

chrome/common/extensions/extension_messages.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // IPC messages for extensions.
 // Multiply-included message file, hence no include guard.
 
 #include <string>
 #include <vector>
 
@@ skipping 99 matching lines @@
 
   // When to inject the code.
   IPC_STRUCT_MEMBER(int, run_at)
 
   // Whether to execute code in the main world (as opposed to an isolated
   // world).
   IPC_STRUCT_MEMBER(bool, in_main_world)
 
   // Whether the request is coming from a <webview>.
   IPC_STRUCT_MEMBER(bool, is_web_view)
+
+  // Whether the caller is interested in the result value. Manifest-declared
+  // content scripts and executeScript() calls without a response callback
+  // are examples of when this will be false.
+  IPC_STRUCT_MEMBER(bool, wants_result)
 IPC_STRUCT_END()
 
 // Struct containing the data for external connections to extensions. Used to
 // handle the IPCs initiated by both connect() and onConnect().
 IPC_STRUCT_BEGIN(ExtensionMsg_ExternalConnectionInfo)
   // The ID of the extension that is the target of the request.
   IPC_STRUCT_MEMBER(std::string, target_id)
 
   // The ID of the extension that initiated the request. May be empty if it
   // wasn't initiated by an extension.
@@ skipping 504 matching lines @@
 // Notifies the browser process that a tab has started or stopped matching
 // certain conditions.  This message is sent in response to several events:
 //
 // * ExtensionMsg_WatchPages was received, updating the set of conditions.
 // * A new page is loaded.  This will be sent after ViewHostMsg_FrameNavigate.
 //   Currently this only fires for the main frame.
 // * Something changed on an existing frame causing the set of matching searches
 //   to change.
 IPC_MESSAGE_ROUTED1(ExtensionHostMsg_OnWatchedPageChange,
                     std::vector<std::string> /* Matching CSS selectors */)