Rename WebSocketChannel to MainThreadWebSocketChannel.

WebSocketChannel is more general name than its the name of its base class, ThreadableWebSocketChannel.
This is a refactoring without behavior change.

Patch from Yutaka Hirano <yhirano@chromium.org>
BUG=230294

Review URL: https://codereview.chromium.org/13860004

Patch from Yutaka Hirano <yhirano@chromium.org>.

git-svn-id: svn://svn.chromium.org/blink/trunk@148432 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Content Security Policy tests should use the unprefixed header.

We used to require the prefixed header for experimental bits and pieces of
the 1.1 spec. We've since changed direction, preferring the canonical
header, and hiding the unstable functionality behind a runtime flag. There's
no longer any reason at all to use 'X-WebKit-CSP'. In order to more clearly
deprecate it, this patch changes all the tests to use the canonical header.

Review URL: https://codereview.chromium.org/14046027

git-svn-id: svn://svn.chromium.org/blink/trunk@148433 bbb929c8-8fbe-4397-9dbb-9b2b20218538