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This re-enables just the browser_tests suite on the
chromium.mac bots and the matching trybots. The tests
on Mac10.13 Tests are still marked as experimental, so
failures there won't block the CQ. The re-enabled tests
will run in the 'Chrome-quarantine' pool, which means
that if the tests kill the machines, it should only
affect the machines in that pool, and not the machines
in the main 'Chrome' pool.

The tests are still disabled on chromium.clang, chromium.fyi,
and chromium.memory, and viz_browser_tests and
surface_sync_browser_tests are still disabled everywhere.

If it looks like this approach is working, we can reenable
those as well as long as they go into the quarantine pool.

R=jbudorick@chromium.org
BUG=828031
NOTRY=true

Change-Id: I2e914b68584fd4d718aba246906f38b0724a08f1
Reviewed-on: https://chromium-review.googlesource.com/1103796
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567922}