Introduce blink_testing GYP target

This CL introduces the blink_testing GYP target so that GYP files in Chromium
can pull in TestRunner without needing to mention any files outside of the
Blink public directory.

R=jochen

Review URL: https://chromiumcodereview.appspot.com/17829002

git-svn-id: svn://svn.chromium.org/blink/trunk@153056 bbb929c8-8fbe-4397-9dbb-9b2b20218538