Adds option to run browser tests in mash

At a high level this is what the code does:
. The test launcher creates MojoTestConnector.
. MojoTestConnector creates a BackgroundShell (initially I wanted to
  run the shell on the same thread, but that is problematic because I
  need to run a nested message loop at times, which the IO thread
  doesn't support (the test launcher uses an io thread)).
. A connection is established to mojo:mash_shell.
. MojoTestConnector sets up the state for each test. The connect is not done
  with Mojo's child process connection. The base test launching code handles
  the actual process launching.

Other random notes:
. --single_process is a slightly different flow.
. The test never finishes. This is because apps that are started don't
  all exit when the shell exits. I'm going to look at that next.

R=ben@chromium.org, jam@chromium.org
BUG=581733

Review URL: https://codereview.chromium.org/1806353003

Cr-Commit-Position: refs/heads/master@{#382305}

0 files changed, 0 insertions, 0 deletions