Add Mojo Environment RunLoop functions; update apptest.

Add [Instantiate|Destroy]DefaultRunLoop() helpers.
Implement their pure Mojo environment versions.
Update ApplicationTestBase and its MojoMain.

TODO: Implement the Chromium-environment versions.

BUG=392646
TEST="mojo_shell mojo:example_apptests" still works.
R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/687753003