This replaces three passes over the roots with a single pass.
This also removes root synchronization logic.

The GC subroot index is computed from the |root| parameter of the visit
method. The new |description| parameter is used as an edge name.

Bug: chromium:811842
Change-Id: I03a9215d56b54b3eb5f7bc8b32d5b22ad091c68b
Reviewed-on: https://chromium-review.googlesource.com/916781Reviewed-by: Alexei Filippov <alph@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51350}