The concurrent marker uses ObjectVisitor to iterate pointers in objects
and local marking bitmaps to keep track of visited objects.

To keep it simple for now, I removed support for multiple tasks
and canceling unfinished tasks.

BUG=chromium:694255

Review-Url: https://codereview.chromium.org/2732053002
Cr-Commit-Position: refs/heads/master@{#43618}