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Run headless_browsertests, headless_unittests on the the clang tot bots,
and on some memory bots. Both suites are very leaky, so they don't run on
asan bots, and headless_browsertests is also very racy so it doesn't run under
tsan, but headless_unittests runs under tsan. Both run under msan and cfi.
They also run under asan/mac, which seems to be happy.

This is part of an effort to reduce the number of test configurations.
Originally reviewed as part of https://chromium-review.googlesource.com/1061795,
which I'm now landing in smaller pieces.

TBR=kbr

Bug: 843511,843732,843734
Change-Id: I3ba57446830d44f3651100e216f1d546ee7532b6
Reviewed-on: https://chromium-review.googlesource.com/1088754
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564931}