[telemetry] Ensure gsutil is a file.

BUG=338780
TBR=achuith@chromium.org

Review URL: https://codereview.chromium.org/149233002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247528 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247528