Pepper proxy: make the browser sender handle the proxy lock properly.

ppapi/proxy/plugin_proxy_delegate.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.
 
 #ifndef PPAPI_PROXY_PLUGIN_PROXY_DELEGATE_H_
 #define PPAPI_PROXY_PLUGIN_PROXY_DELEGATE_H_
 
 #include <string>
 
 namespace IPC {
 class Sender;
 }
 
 namespace ppapi {
 namespace proxy {
 
 class PPAPI_PROXY_EXPORT PluginProxyDelegate {
  public:
   virtual ~PluginProxyDelegate() {}
 
-  // Sends the given message to the browser. Identical semantics to IPC::Sender
-  // interface. New code should use GetBrowserSender instead.
-  // TODO(brettw) remove this.
-  virtual bool SendToBrowser(IPC::Message* msg) = 0;
-
   // Returns the channel for sending to the browser.
+  // Note: The returned sender must be thread-safe. It might be used while the
+  // proxy lock is not acquired. Please see the implementation of
+  // PluginGlobals::BrowserSender.
   virtual IPC::Sender* GetBrowserSender() = 0;
 
   // Returns the language code of the current UI language.
   virtual std::string GetUILanguage() = 0;
 
   // Performs Windows-specific font caching in the browser for the given
   // LOGFONTW. Does nothing on non-Windows platforms.
+  // Note: This method must be thread-safe.
   virtual void PreCacheFont(const void* logfontw) = 0;
 
   // Sets the active url which is reported by breakpad.
   virtual void SetActiveURL(const std::string& url) = 0;
 };
 
 }  // namespace proxy
 }  // namespace ppapi
 
 #endif  // PPAPI_PROXY_PLUGIN_PROXY_DELEGATE_H_