Use xvfb with Ozone CrOS.

We want to switch the majority of Ozone CrOS tests over to run with
--ozone-platform=x11. This requires that test targets with type
'windowed_test_launcher' run using xvfb.

Right now mb.py looks for the arg '--ozone-platform=x11' in the isolate
step, which requires adding a new target for each test we want to add.
Instead, just assume that if use_ozone=true and target_os="chromeos"
then we want to use xvfb.

Ozone CrOS test with type 'windowed_test_launcher' will still need to
have '--ozone-platform=x11' flag added in chromimum.chromiumos.json to
run with Ozone X11. Once most tests pass under with Ozone X11 the
default Ozone platform can be swapped to be x11 and only tests that
should run with headless will require an args entry there.

BUG=671355

Review-Url: https://codereview.chromium.org/2587473002
Cr-Commit-Position: refs/heads/master@{#439557}
4 files changed