Don't rely on webrtc/base/scoped_ptr.h to include stuff for you

webrtc/base/scoped_ptr.h doesn't need to include webrtc/base/common.h
anymore, but a couple of its users were relying on it to pull in other
things for them. Fix that, and remove the now really unnecessary
webrtc/base/common.h include.

R=andrew@webrtc.org, pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/37169004

Cr-Commit-Position: refs/heads/master@{#8333}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8333 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/base/buffer.h

@@ -13,6 +13,7 @@
 
 #include <string.h>
 
+#include "webrtc/base/common.h"
 #include "webrtc/base/scoped_ptr.h"
 
 namespace rtc {