Update {virtual,override} to follow C++11 style in ppapi.

ppapi/proxy/proxy_channel.h

Index: ppapi/proxy/proxy_channel.h
diff --git a/ppapi/proxy/proxy_channel.h b/ppapi/proxy/proxy_channel.h
index 97069beb36723ecffb3fbbed2edd3bb46a8a320d..ffa906ee3e60bde735b26280e296ab9540cb3301 100644
--- a/ppapi/proxy/proxy_channel.h
+++ b/ppapi/proxy/proxy_channel.h
@@ -52,7 +52,7 @@ class PPAPI_PROXY_EXPORT ProxyChannel
         bool should_close_source) = 0;
   };
 
-  virtual ~ProxyChannel();
+  ~ProxyChannel() override;
 
   // Alternative to InitWithChannel() for unit tests that want to send all
   // messages sent via this channel to the given test sink. The test sink
@@ -71,10 +71,10 @@ class PPAPI_PROXY_EXPORT ProxyChannel
       bool should_close_source);
 
   // IPC::Sender implementation.
-  virtual bool Send(IPC::Message* msg) override;
+  bool Send(IPC::Message* msg) override;
 
   // IPC::Listener implementation.
-  virtual void OnChannelError() override;
+  void OnChannelError() override;
 
   // Will be NULL in some unit tests and if the remote side has crashed.
   IPC::SyncChannel* channel() const {