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}