Add blink_web_test_support target to blink.gyp

That target will replace the blink_test_support target in
public/blink_test_runner.gyp. After this lands, chromium will be updated
to use this new target, and just after chromium switches to it we will
be able to remove blink_test_runner.gyp.

test_runner is a misleading name for this gyp because there is no
test_runner in Blink anymore, this was moved into
content/shell/renderer/test_runner in Chromium.

BUG=None
TEST=None
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/158243002

git-svn-id: svn://svn.chromium.org/blink/trunk@166798 bbb929c8-8fbe-4397-9dbb-9b2b20218538