Add the concept of a "smoke tests" list to the layout tests.

This is a list of tests that runs quickly and gets decent coverage. The
goal is to run approximately 1% of the tests, or enough so that the
slowest ports can still run a full test cycle in 10 minutes or less.

The list of tests will be found in LayoutTests/SmokeTests. The list is
a placeholder for now, but will ultimately be generated by a script. You
can run the test list with the command line argument --smoke (more or less
equivalent to --test-list=.../LayoutTests/SmokeTests), and ports can
specify that, if no tests are given, whether to run only the SmokeTests
by default (e.g., Android will use this). Lastly, you can specify --no-smoke
to *not* automatically run the smoke test (and instead act as if --test-list=...
was not being appended to the command line).

R=peter@chromium.org, abarth@chromium.org, ojan@chromium.org
BUG=289883

Review URL: https://chromiumcodereview.appspot.com/23509012

git-svn-id: svn://svn.chromium.org/blink/trunk@157878 bbb929c8-8fbe-4397-9dbb-9b2b20218538