Use separate visitors for roots visitation:
- A general RootVisitor for all regular roots
- An ObjectVisitor that is able to process contents of the string table
  and code hanging of the top optimized frame

Bug: chromium:750084
Change-Id: I21aaa18760dede8a9419de2890a47c6dcf7efa3b
Reviewed-on: https://chromium-review.googlesource.com/598239Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47089}