Do not try to stop org.chromium.content_shell_apk if we did not start it.

TBR=peter@chromium.org
BUG=305040

Review URL: https://codereview.chromium.org/68013010

git-svn-id: svn://svn.chromium.org/blink/trunk@162321 bbb929c8-8fbe-4397-9dbb-9b2b20218538