Slow down incremental marking on main thread when idle notification is active.

BUG=

Review URL: https://codereview.chromium.org/771703004

Cr-Commit-Position: refs/heads/master@{#25630}
Print finalize incremental marking event in idle notification.

BUG=

Review URL: https://codereview.chromium.org/775013002

Cr-Commit-Position: refs/heads/master@{#25631}
[turbofan] Initial work on cleaning up the Node class.

- Remove use_count_, only used for debug builds and tests.
- Use BitField instead of the unreliable C++ bit fields.
- Improve memory layout.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/780503002

Cr-Commit-Position: refs/heads/master@{#25632}