Add ALLOW_UNUSED and update COMPILE_ASSERT to Chromium's latest.

Fixes building with gcc 4.8.

TBR=fdegans@google.com
BUG=chromium:321833

Review URL: https://webrtc-codereview.appspot.com/12439004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6050 4adac7df-926f-26a2-2b94-8c16560cd09d