[WebSocket] Remove unused method in wrapper class

ThreadableWebSocketChannelClientWrapper::processPendingTasksCallback is
no longer used. Remove it.

BUG=none

Review URL: https://codereview.chromium.org/195873003

git-svn-id: svn://svn.chromium.org/blink/trunk@169093 bbb929c8-8fbe-4397-9dbb-9b2b20218538