Revert "[oilpan]: Make thread shutdown more robust."

Reverts https://codereview.chromium.org/371623002 and
https://codereview.chromium.org/394663005

Sorry to revert this change. It likely caused failures on the indexdb tests
listed in the bug, and falls in the revision range of unsolved mystery
crbug.com/393706 (as unlikely as it looks to have caused it).


BUG=393832,393706
TBR=wibling,tkent
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/394733002

git-svn-id: svn://svn.chromium.org/blink/trunk@178142 bbb929c8-8fbe-4397-9dbb-9b2b20218538