GitLab

Reparents the tab grid to SceneController. Note that the lazy getter
loses a DCHECK - it seems too hard to cleanly port to SceneController
at the moment, though once multiwindow is operational we should add new
invariant checks everywhere.

Bug: 1045657
Change-Id: I7e4eecd58bea4338a634f9e01e2cbc255c49c328
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2087337
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747694}