Add a DEPS file to Blink to control the revisions of the w3c test repos.

This is patch 1 of 6-ish that will change how we import and run the
w3c test suites, moving the DEPS entries from Chromium's src/DEPS to
a new DEPS file in Blink. Once this whole process is complete,
importing new versions of the w3c repos will require only a single change
to Blink (as opposed to the up-to-5-sided patches required currently).

Full details are in crbug.com/395862.

R=eseidel@chromium.org
BUG=395862
NOTRY=true

Review URL: https://codereview.chromium.org/410803002

git-svn-id: svn://svn.chromium.org/blink/trunk@178713 bbb929c8-8fbe-4397-9dbb-9b2b20218538