Use override keyword in ppapi::proxy::InterfaceProxy.

Instead of virtual, since it inherits from IPC::Sender.
Do the same for child classes and fix lint errors.

Change-Id: Ie01bdd94d0606ce5d2f9e38e08707224c477ac91
Reviewed-on: https://chromium-review.googlesource.com/642461
Commit-Queue: Lei Zhang <[email protected]>
Reviewed-by: Bill Budge <[email protected]>
Cr-Commit-Position: refs/heads/master@{#498678}
diff --git a/ppapi/proxy/ppb_message_loop_proxy.h b/ppapi/proxy/ppb_message_loop_proxy.h
index f78c3772..9c15539 100644
--- a/ppapi/proxy/ppb_message_loop_proxy.h
+++ b/ppapi/proxy/ppb_message_loop_proxy.h
@@ -121,7 +121,7 @@
 class PPB_MessageLoop_Proxy : public InterfaceProxy {
  public:
   explicit PPB_MessageLoop_Proxy(Dispatcher* dispatcher);
-  virtual ~PPB_MessageLoop_Proxy();
+  ~PPB_MessageLoop_Proxy() override;
 
   static const PPB_MessageLoop_1_0* GetInterface();