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
diff --git a/webrtc/base/httprequest.cc b/webrtc/base/httprequest.cc
index 9ce2377..3199c8f 100644
--- a/webrtc/base/httprequest.cc
+++ b/webrtc/base/httprequest.cc
@@ -54,6 +54,8 @@
       client_(user_agent.c_str(), NULL), error_(HE_NONE) {
 }
 
+HttpRequest::~HttpRequest() = default;
+
 void HttpRequest::Send() {
   // TODO: Rewrite this to use the thread's native socket server, and a more
   // natural flow?