On left trimming the main thread ensures that the array
is black before changing its map and length.

The concurrent marker snapshots the length of the array
and then tries to change its color to black. If the change
is successful, then the array is iterated using the saved
length.


BUG=chromium:694255

Change-Id: Ib06c1c9c3240ace56c1897e5a73462aa447d41f5
Reviewed-on: https://chromium-review.googlesource.com/519323Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45656}