Copy DatabaseTask.{h,cpp} to TaskSynchronizer.{h,cpp}

This CL just copies a file just to make the diff of
https://codereview.chromium.org/198673002/ saner.

TBR=tkent
NOTRY=true

Review URL: https://codereview.chromium.org/220703002

git-svn-id: svn://svn.chromium.org/blink/trunk@170531 bbb929c8-8fbe-4397-9dbb-9b2b20218538