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/taskrunner.h

@@ -27,7 +27,7 @@ const int64 kSecTo100ns = kSecToMsec * kMsecTo100ns;
 class TaskRunner : public TaskParent, public sigslot::has_slots<> {
  public:
   TaskRunner();
-  virtual ~TaskRunner();
+  ~TaskRunner() override;
 
   virtual void WakeTasks() = 0;