[iOS] Close thumb strip when tapping on grid background.

Adds a gesture recognizer to the tab grid background. When tapped, this
will cause the thumb strip to dismiss. The recognizer is disabled when
switching to grid mode, and uninstalled when the thumb strip is disabled.

To prevent taps between cells from triggering this, the recognizer
ignores taps that are to the right of the last cell in the thumb strip
(or to the left in RTL). A small helper is added to rtl_geometry for
this.

To be able to dismiss the thumb strip, a handler had to be plumbed into
the GridViewController, which meant renaming the existing 'handler'
property to 'reauthHandler'.

The bulk of the other additions to this CL are added to make an EG test
for this change possible:

 - Accessibility IDs are added for the tab grid background, and for each
   of the two tab grids.
 - New matchers are added for these IDs.
 - An isRTL helper is added so the eg test can tap in the right location.

The EG test itself is straightforward.

Bug: 1171367
Change-Id: I663198bca91057eac10f6c1e75daf1d79c47c12f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2705731


Commit-Queue: Mark Cogan <marq@chromium.org>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#864947}