Fix clang style warnings in webrtc/base

Mostly this consists of marking functions with override when
applicable, and moving function bodies from .h to .cc files.

Not inlining virtual functions with simple bodies such as

  { return false; }

strikes me as probably losing more in readability than we gain in
binary size and compilation time, but I guess it's just like any other
case where enabling a generally good warning forces us to write
slightly worse code in a couple of places.

BUG=163
R=kjellander@webrtc.org, tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/47429004

Cr-Commit-Position: refs/heads/master@{#8656}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8656 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/base/asyncresolverinterface.h

@@ -19,8 +19,8 @@ namespace rtc {
 // This interface defines the methods to resolve the address asynchronously.
 class AsyncResolverInterface {
  public:
-  AsyncResolverInterface() {}
-  virtual ~AsyncResolverInterface() {}
+  AsyncResolverInterface();
+  virtual ~AsyncResolverInterface();
 
   // Start address resolve process.
   virtual void Start(const SocketAddress& addr) = 0;