Get rid of runInMode from WorkerThreadableWebSocketChannel

- Uses blink::WebWaitableEvent instead for synchronous methods
- Some setters/getters in ThreadableWebSocketChannelClientWrapper are extracted into another small class, ThreadableWebSocketChannelClientSyncHelper, which is a dedicated class for sync method calls

BUG=340249
TEST=http/tests/websocket/workers/*

Review URL: https://codereview.chromium.org/132913009

git-svn-id: svn://svn.chromium.org/blink/trunk@166487 bbb929c8-8fbe-4397-9dbb-9b2b20218538