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

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6636 4adac7df-926f-26a2-2b94-8c16560cd09d