mock_platform: fix argument count for GetGroupId mock

It said 3 args but it really only needs 2.

TEST=unittests compile and pass. They didn't before
BUG=none

Change-Id: I68d7de591aa24159669bf3d8ee5d49e787595424

R=gauravsh@chromium.org,davidjames@chromium.org

Review URL: http://codereview.chromium.org/6771026
1 file changed