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Previously headless ozone platform was used on Fuchsia. Switched it to
scenic. content_unittests fail with scenic, so this change also adds
--ozone-platforms=headless flag in the unittests, until this issue
is resolved.

Bug: 829980, 865172
Change-Id: I8a0b282953cf90554dc78ed1575d927b53374c58
Reviewed-on: https://chromium-review.googlesource.com/1142492Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576360}