After 52e8d0ab incremental marking observer is invoked ~8 times more
often than before. This patch increases the allocation observer
threshold for incremental marking and scales the step size based
on the number of concurrent marking tasks.

Bug: chromium:768664
TBR: mlippautz@chromium.org
Change-Id: I0afd5dccd55f32c7f545d9c3a47edc20c6fd83db
Reviewed-on: https://chromium-review.googlesource.com/683955Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48154}