[WebSocket] Remove "Threadable" from class names.

"Threadable" in class names came from core/loader (e.g. ThreadableLoader),
but it doesn't have any meaning in modules/websockets. This CL drops
that adjective from class names.

R=tyoshino,mkwst
BUG=NONE

Review URL: https://codereview.chromium.org/701753002

git-svn-id: svn://svn.chromium.org/blink/trunk@184835 bbb929c8-8fbe-4397-9dbb-9b2b20218538