This adds the unittests to the "default" test set. Now that
the "default" and the DEFAULT_TESTS (i.e. runner with
no arguments) are the same, removed DEFAULT_TESTS and use
TEST_MAP["default"] instead.

On the bots, where unittests and default were run in
separation before, the explicit unittests step should now
be skipped.

This is necessary for swarming, as the unittests step is
too small to justify its own swarming job.

BUG=chromium:535160
LOG=n

Review URL: https://codereview.chromium.org/1374733006

Cr-Commit-Position: refs/heads/master@{#31122}