Make GCTracer not reentrant.

BUG=

Review URL: https://codereview.chromium.org/754023003

Cr-Commit-Position: refs/heads/master@{#25524}
Reuse CFGBuilder in the scheduler to save memory.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/755353003

Cr-Commit-Position: refs/heads/master@{#25525}
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}