Rename webkit_test_support target to blink_web_test_support.

Now that Chromium has been updated to use blink_test_support, we can
remove blink_web_test_support target from blink.gyp and rename
webkit_test_support to blink_web_test_support.

BUG=None
TEST=None. No functional changes.
R=dpranke@chromium.org
TBR=abarth

Review URL: https://codereview.chromium.org/293033011

git-svn-id: svn://svn.chromium.org/blink/trunk@174714 bbb929c8-8fbe-4397-9dbb-9b2b20218538