Linux Sandbox: Stop GPU watchdog in accountable way.

The Linux sandbox can sometimes detect a spurious running thread if it
has just been stopped.

We add a new LinuxSandbox::StopThread() method to safely stop threads
and make sure they won't be counted as still running.

BUG=328620
NOTRY=true

Review URL: https://codereview.chromium.org/147203005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249937 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249937