Disable MediaGalleriesPrivateApiTest.DeviceAttachDetachEvents on Mac because it's flaky on the trybots.

BUG=326324
TBR=thestig

Review URL: https://codereview.chromium.org/107373002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239058 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239058