[WebSocket] Enable permessage-deflate by default.

Now permessage-deflate is enabled by default.
Fix failing tests and remove flag manipulation in tests.

BUG=229290
R=tyoshino

Review URL: https://codereview.chromium.org/27576005

git-svn-id: svn://svn.chromium.org/blink/trunk@160077 bbb929c8-8fbe-4397-9dbb-9b2b20218538