customtabs: Run the session cleanup code on the UI thread.

customtabs: Run the session cleanup code on the UI thread.

This fixes two issues, a test issue and a real one:
- CustomTabsConnectionTest#tearDown() doesn't run on the UI thread. It
  calls CustomTabsConnection#cleanupAll() which calls a method that has
  to run on the UI thread. Run CustomTabsConnectionTest#tearDown() on
  the UI thread to fix this.
- IBinder.DeathRecipient#binderDied() is not necessarily called on the
  UI thread, and has to unregister the death recipient.

BUG=523057
Committed: https://crrev.com/e26acf52b32b3e49ceec2b58f0e86cc932b9715b
Cr-Commit-Position: refs/heads/master@{#344797}

[Benoit L, 2015-08-21 09:50:22]

lizeb@chromium.org changed reviewers:
+ pasko@chromium.org, yusufo@chromium.org

[pasko, 2015-08-21 12:06:55]

good catch! lgtm

[Yusuf, 2015-08-21 16:52:24]

lgtm.

Thanks Benoit!

[Benoit L, 2015-08-21 16:55:21]

The CQ bit was checked by lizeb@chromium.org

[commit-bot: I haz the power, 2015-08-21 16:55:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1304963004/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1304963004/40001

[commit-bot: I haz the power, 2015-08-21 17:09:31]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-08-21 17:10:14]

Patchset 3 (id:??) landed as
https://crrev.com/e26acf52b32b3e49ceec2b58f0e86cc932b9715b
Cr-Commit-Position: refs/heads/master@{#344797}