Disable GpuFeatureTest.MultisamplingAllowed

This test was basically only running when on OSMesa and is disabled in
many cases. Just disable the test officially until it can be enabled
more cleanly.

R=zmo@chromium.org
BUG=270918

Review URL: https://codereview.chromium.org/178493002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252985 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252985