Kill worker process by way of a sync IPC message before it cleans up.

When a worker process shuts down, it shuts down WebKit. If there are
other threads running in the worker process, this leads to crashes.
This fix tries to kill the worker process forcibly so that no cleanup
takes place.

BUG=249502

Review URL: https://codereview.chromium.org/23496052

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233099 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233099