Automatically reauthenticate to Rietveld
Automatically reauthenticate to Rietveld

Added automatic detection of OAuth2 token expiracy. Token is
automatically renewed when necessary.

BUG=363825

Review URL: https://codereview.chromium.org/243323003

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@264871 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools