Implement full duplex Mojo connection on ConnectionHolder.

Also, applies it to AppInstance/AppHost connection, as a first
use case.
Remaining will be done in following CLs.

BUG=750563
TEST=Ran bots.

Change-Id: Ie312d6b6f2421742fbd6f1db0dd75d8db5ea3bb6
Reviewed-on: https://chromium-review.googlesource.com/776742


Reviewed-by: Luis Hector Chavez <lhchavez@chromium.org>
Reviewed-by: Yury Khmel <khmel@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518270}