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

git-svn-id: svn://svn.chromium.org/blink/trunk@170532 bbb929c8-8fbe-4397-9dbb-9b2b20218538