Remove worker support of Web SQL Database.

Intent to remove:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/SYlD0YVUnQk

Remove all *Sync interfaces and implementations, and WorkerGlobalScope::
openDatabase.
CLs to simplify code follow.

BUG=397429,323858,310866,357427

Review URL: https://codereview.chromium.org/561093003

git-svn-id: svn://svn.chromium.org/blink/trunk@181790 bbb929c8-8fbe-4397-9dbb-9b2b20218538