Add a few missing overrides found by a new clang warning.

Namely, -Winconsistent-missing-override. No behavior change.

BUG=428099

Review URL: https://codereview.chromium.org/687693004

Cr-Commit-Position: refs/heads/master@{#301746}

diff --git a/ipc/ipc_channel.h b/ipc/ipc_channel.h
index f077def..2fc52cb5 100644
--- a/ipc/ipc_channel.h
+++ b/ipc/ipc_channel.h

@@ -173,11 +173,12 @@
   // Get its own process id. This value is told to the peer.
   virtual base::ProcessId GetSelfPID() const = 0;
 
+  // Overridden from ipc::Sender.
   // Send a message over the Channel to the listener on the other end.
   //
   // |message| must be allocated using operator new.  This object will be
   // deleted once the contents of the Message have been sent.
-  virtual bool Send(Message* message) = 0;
+  virtual bool Send(Message* message) override = 0;
 
   // NaCl in Non-SFI mode runs on Linux directly, and the following functions
   // compiled on Linux are also needed. Please see also comments in