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

The Google style guide states that only one of {virtual,override,final} should be used for each declaration, since override implies virtual and final implies both virtual and override.

This patch was manually generated using a regex and a text editor.

BUG=417463

Review URL: https://codereview.chromium.org/1097393007

Cr-Commit-Position: refs/heads/master@{#326505}
diff --git a/ppapi/proxy/ppp_messaging_proxy.h b/ppapi/proxy/ppp_messaging_proxy.h
index 90b674c..f232c6b 100644
--- a/ppapi/proxy/ppp_messaging_proxy.h
+++ b/ppapi/proxy/ppp_messaging_proxy.h
@@ -18,10 +18,10 @@
 class PPP_Messaging_Proxy : public InterfaceProxy {
  public:
   PPP_Messaging_Proxy(Dispatcher* dispatcher);
-  virtual ~PPP_Messaging_Proxy();
+  ~PPP_Messaging_Proxy() override;
 
   // InterfaceProxy implementation.
-  virtual bool OnMessageReceived(const IPC::Message& msg) override;
+  bool OnMessageReceived(const IPC::Message& msg) override;
 
  private:
   // Message handlers.