Fix CAP composited selection for empty selection boundaries

The code wasn't handling cases where the start of end of the selection
is empty. In those cases, we were not recording selection bounds as the
selection is not painted on those lines so the handles disappear.

Check explicitly for this and use this as a trigger for whether or not
to record selection bounds. This CL also shuffles a few existing tests
around and adds a new test to cover both legacy and NG codepaths.

Bug: 1238256
Change-Id: Icd5b57ff4aacb2101c55ec60cb9e8e110e7678c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3088074


Commit-Queue: Daniel Libby <dlibby@microsoft.com>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#911211}