Add blink_test_support target.

This target will replace the current blink_web_test_support target. But
we need to add it first, then change Chromium to use, just after that we
will be able to remove blink_web_test_support from blink.gyp.

In fact webkit_test_support after this three side change lands,
webkit_test_support will actually be renamed to blink_web_test_support.

BUG=None
TEST=content_unittests, blink_tests
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/180743011

git-svn-id: svn://svn.chromium.org/blink/trunk@168315 bbb929c8-8fbe-4397-9dbb-9b2b20218538