Chrome browser_tests often assume they can make synchronous in-process
calls into ash to check UI state. This won't work under mash. These
interfaces often access private state, so we don't want to expose them
to non-test code.

Design doc at go/mash-test-interfaces

* Create a separate build target for test mojo interfaces
* Only bind the interfaces when running a test suite
* Add a mojo::test::Waiter that allows tests to be written in the
  synchronous style without using sync mojo IPC (kudos to rockot@
  for writing this)
* Convert two browser tests as examples

The tests now run under --mash without crashing. One fails, which is
expected because it relies on a VPN feature that hasn't been converted
to mash.

Bug: none
Test: browser_tests and browser_tests --mash
Change-Id: If4bbeb32e58367d41e3ae5b8689eb704f864a960
Reviewed-on: https://chromium-review.googlesource.com/702636


Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508101}