GitLab

Move CustomTabSessionImpl to its own file. Also, factor out code that
can be used by other classes that create and manage an ARC Custom Tab.

BUG=b:64863368
TEST=Deploy Chrome to DUT and verify ARC Custom Tabs work correctly
R=djacobo@chromium.org, hashimoto@chromium.org, oshima@chromium.org

Change-Id: Ib11a233aa04ac52cfe73abaa9ecbb145791aa263
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1728446Reviewed-by: David Jacobo <djacobo@chromium.org>
Reviewed-by: Mitsuru Oshima (Slow - traveling) <oshima@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Commit-Queue: Jesse Schettler <jschettler@chromium.org>
Auto-Submit: Jesse Schettler <jschettler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683483}