First checked-in import of the W3C's test suites.

The test suites are moving from being pulled in via src/DEPS to
LayoutTests/w3c to being checked in directly to LayoutTests/imported.

This import uses update-w3c-deps run manually and
imported csswg-test@6ed3bb6642b0d382ce8c3f7912172f2420eb7756 and
imported web-platform-tests@6ed3bb6642b0d382ce8c3f7912172f2420eb7756

In addition, we modify run-webkit-tests to manually skip the old
LayoutTests/w3c directory if it exists, and to start printing warnings
that the repos are stale and should be deleted.

This is patchset 3 of the 4 needed to complete the cutover.

TBR=eseidel@chromium.org
BUG=413454

Review URL: https://codereview.chromium.org/560893005

git-svn-id: svn://svn.chromium.org/blink/trunk@181874 bbb929c8-8fbe-4397-9dbb-9b2b20218538