cros_platform_backend.GetCommandLine implementation.

BUG=325604
TEST=manual
NOTRY=True

Review URL: https://codereview.chromium.org/105483002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238812 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238812