Abstract away index-based lookups in TabDragController

This CL updates the TabDragContext interface to support tab lookups via
WebContents and adds a general "IsTabDetachable" method. These new
methods replace TabDragController's usage of GetTabAt and GetIndexOf,
which will allow other implementors (specifically Vertical tab strip),
to use a tab model structure that isn't flat.

Note: GetTabAt still exists, but is only used for positioning logic that does not need to be implemented  for Vertical tabs. A follow-up CL will factor out these unnecessary methods.

Bug: 439963720
Change-Id: I405d4ec27cddcda3c78bda285ddbe06e9d078e2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7247988
Commit-Queue: Kaan Alsan <alsan@chromium.org>
Reviewed-by: David Pennington <dpenning@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1557750}