The reason we need this mode is that IterateRoots for the Scavenger only
captures dependent weak nodes. This is also what we do for marking for the
minor MC.

Since the regular marking might also mark objects that are weakly
(non-dependently) pointed to by nodes we need to capture all of them during
pointers updating. The reason this works for the Scavenger is because we do one
pass at the end of the scavenger (combined with resetting) that captures all
those nodes.

BUG=chromium:651354

Review-Url: https://codereview.chromium.org/2869413002
Cr-Commit-Position: refs/heads/master@{#45248}