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Add mojo/example/apptest with a service and client.
Add example_apptest.cc with a GTest Mojo application.

These tests can be run from out/<Debug|Release>/ via:
./mojo_shell --origin=file://`pwd` --disable-cache mojo:mojo_example_apptests

BUG=392646
TEST=The new mojo_example_apptests passes.
R=ben@chromium.org

Review URL: https://codereview.chromium.org/399653004

Cr-Commit-Position: refs/heads/master@{#288250}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288250 0039d316-1c4b-4281-b951-d872f2087c98