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

ppapi/proxy/plugin_proxy_delegate.h

Index: ppapi/proxy/plugin_proxy_delegate.h
diff --git a/ppapi/proxy/plugin_proxy_delegate.h b/ppapi/proxy/plugin_proxy_delegate.h
index de0c397eba80743ca26c079903fa54c94dac7d09..f2d7759918c4ac41233220388914fb62ad666836 100644
--- a/ppapi/proxy/plugin_proxy_delegate.h
+++ b/ppapi/proxy/plugin_proxy_delegate.h
@@ -18,12 +18,10 @@ 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.
@@ -31,6 +29,7 @@ class PPAPI_PROXY_EXPORT PluginProxyDelegate {
 
   // 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.