[websql] Remove DatabaseThread::scheduleImmediateTask

It's a horrible API. We shouldn't use it.

BUG=301514
R=michaeln@chromium.org

Review URL: https://codereview.chromium.org/83613002

git-svn-id: svn://svn.chromium.org/blink/trunk@162589 bbb929c8-8fbe-4397-9dbb-9b2b20218538