Push roll candidates based on candidate ref instead of lkgr.

Push roll candidates based on candidate ref instead of lkgr.

This pushes v8 to the candidates branch based on the
candidate ref, which is guaranteed to be the same for 8
hours.

The candidate ref is maintained by this bot:
http://build.chromium.org/p/client.v8/builders/Auto-roll%20-%20release%20process

NOTRY=true
BUG=chromium:431669
LOG=n

Review URL: https://codereview.chromium.org/760793003

Cr-Commit-Position: refs/heads/master@{#25526}