Add PhysicalTestSuites to allow adding command line flags for a dir.

This allows us to enable force-compositing-mode on a single directory
so that we can parallelize the work of enabling it, and break the
change to turn it on into smaller bitesize pieces of review and
rebaseline.

R=dpranke@chromium.org, enne
BUG=361729

Review URL: https://codereview.chromium.org/267083002

git-svn-id: svn://svn.chromium.org/blink/trunk@173441 bbb929c8-8fbe-4397-9dbb-9b2b20218538