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Instead of processing ephemerons during each incremental step on the
mutator thread, this CL establishes the following:
* Epehemerons are fully processed during every concurrent marking step.
* Incremental marking on the mutator thread flushes ephemeron pairs
  every few steps. These steps also performs a single iteration of
  ephemeron processing every few steps (lower frequency than flushing
  to allow concurrent markers to do most of the work). Fixed-point
  processing during these tasks is not considered cost-effective.
* Marking during the atomic pause performs full fixed-point processing.

Bug: 986235, 1099415
Change-Id: Id2980ef467b393b4062c5a1206ac1212233e6658
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2260015Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784417}