Use loops directly in apptest main files.

Use mojo::RunLoop in application_test_main.cc
Use base::MessageLoop in application_test_main_chromium.cc
Avoid Environment's DefaultRunLoop abstraction.

This follows some code review feedback from:
https://codereview.chromium.org/710403002/

BUG=392646
TEST=apptests still build and run fine.
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/718123002