Rename WebSocketTest to DOMWebSocketTest

This CL is a followup of https://codereview.chromium.org/394063002/ .

BUG=NONE
R=tyoshino

Review URL: https://codereview.chromium.org/396863002

git-svn-id: svn://svn.chromium.org/blink/trunk@178245 bbb929c8-8fbe-4397-9dbb-9b2b20218538