Make commit links/descriptions work in sheriff-o-matic post git migration.

-Point chromium revisions to crrev.com.
-Properly parse out git commit positions from the commit description.

BUG=401855
R=hinoka@google.com

Review URL: https://codereview.chromium.org/501773002

git-svn-id: svn://svn.chromium.org/blink/trunk@180831 bbb929c8-8fbe-4397-9dbb-9b2b20218538