[unified-heap] Avoid scheduling empty incremental steps

These steps have overhead that is noticeable. Instead, bail out
after first observing the work list as empty. Newly discovered
objects are still processed incrementally via V8 helping mechanisms.

Bug: 843903
Change-Id: I3567557635418e284b85815e3e83aa14e64310bf
Reviewed-on: https://chromium-review.googlesource.com/c/1269039
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597848}
1 file changed