[WebSocket] Task creation should be separated from task posting.

      Having a complex argument as a waitForMethodCompletion may keep alive
temporary objects which must be killed before posting a task to another
thread.

BUG=390174

Review URL: https://codereview.chromium.org/368453003

git-svn-id: svn://svn.chromium.org/blink/trunk@177359 bbb929c8-8fbe-4397-9dbb-9b2b20218538