Revert 6637 "Revert 6636 "Roll chromium_revision 280876:281479""

GN issue should be fixed in http://crrev.com/282138.

> Revert 6636 "Roll chromium_revision 280876:281479"
> 
> Still breaks GN bot.
> 
> > Roll chromium_revision 280876:281479
> > 
> > No significant DEPS changes in this roll, only some changes in how clang_format is downloaded. clang_format changes based on https://webrtc-codereview.appspot.com/20829004 which was reverted.
> > 
> > Review URL: https://webrtc-codereview.appspot.com/19929004
> 
> TBR=henrikg@webrtc.org
> 
> Review URL: https://webrtc-codereview.appspot.com/14909004

TBR=henrikg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/20899004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6643 4adac7df-926f-26a2-2b94-8c16560cd09d