🖿🔬 Move tab observation from LayoutManagerChrome to LayoutManager.

This ensures that all activities using a LayoutManager observe the necessary tab
events (not just ChromeTabbedActivity), and allows removing the
CustomTabLayoutManager, which used to observe only a subset of events.

Bug: 798933
Change-Id: I20242ba89fc058256adeddfdc42260c0220a9fe8
Reviewed-on: https://chromium-review.googlesource.com/904982
Commit-Queue: Bernhard Bauer <bauerb@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535351}