Drop warnings from adb shell output

This is a quick and dirty fix for a problem that occurs to me when
trying to run instrumentation tests in a simulator.
For some reason 'pm path org.chromium.content_shell_apk.tests' outputs
a warning message in my environment that confuses the python scripts
and they end up calling md5sum for the root directory which is a bit
slow... The real fix would be to be able drop the stderr of the command
but it needs a lot of plumbing and probably there is not that much chance
that something like that will happen again for someone.

BUG=334439

Review URL: https://codereview.chromium.org/138143019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247742 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247742