Revert 170532 "Rename DatabaseTaskSynchronizer to TaskSynchroniz..."

I miscommitted r170532 with unnecessary changes.
(r170532 was a sum of https://codereview.chromium.org/198673002 and https://codereview.chromium.org/216203006/,
but I should land only https://codereview.chromium.org/198673002.)
I'll reland those CLs separately.

> Rename DatabaseTaskSynchronizer to TaskSynchronizer and move it to platform/
> 
> - We want to use DatabaseTaskSynchronizer in the HTML parser thread in a follow-up CL. In preparation for that, this CL renames DatabaseTaskSynchronizer to TaskSynchronizer and moves it to platform/.
> 
> BUG=340522
> R=ager@chromium.org, tkent@chromium.org, wibling@chromium.org
> 
> Review URL: https://codereview.chromium.org/198673002

TBR=haraken@chromium.org

Review URL: https://codereview.chromium.org/213673004

git-svn-id: svn://svn.chromium.org/blink/trunk@170534 bbb929c8-8fbe-4397-9dbb-9b2b20218538