Make DatabaseTask inherit from WebTask

The goal is to implement DatabaseThread in terms of WebThread, and
this is a first step into that direction.

BUG=301514
R=michaeln@chromium.org,abarth@chromium.org,kinuko@chromium.org

Review URL: https://codereview.chromium.org/73623004

git-svn-id: svn://svn.chromium.org/blink/trunk@162214 bbb929c8-8fbe-4397-9dbb-9b2b20218538