[turbofan] Add a PhaseScope for initialization.

R=dcarney@chromium.org
BUG=

Review URL: https://codereview.chromium.org/773443002

Cr-Commit-Position: refs/heads/master@{#25587}
Use just one marking deque.

BUG=

Review URL: https://codereview.chromium.org/770453003

Cr-Commit-Position: refs/heads/master@{#25588}