Don't issue Stop() for spurious command line differences.
The android driver was checking self._current_cmd_line against its
own _android_driver_cmd_line() even though ServerProcess.start() was
setting that value based on the normal cmd_line().
Doing so results in the appearance of different command lines which
causes a ServerProcess.stop() to be issued and tear everything down.
_android_driver_cmd_line() = ['ContentShell.apk', '--no-timeout',
'--encode-binary', '--enable-hardware-gpu', '--force-compositing-mode',
'--enable-accelerated-fixed-position', '--dump-render-tree', '-']
_cmd_line() = ['adb', '-s', u'04aa0e7cdd540dac', 'shell']
Now a single content_shell instance is responsible for running all
the tests! Much faster :)
TEST=content_shell tests use a single instance.