Disable tab capture perf tests on Mac.

This was recently re-enabled, but is failing on the Mac GPU bots.

TBR=hubbe@chromium.org
BUG=308236

Review URL: https://codereview.chromium.org/27629002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229015 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229015