Fix native client failures on mac by only using Mesa on Linux.

BUG=125106
R=binji@chromium.org, bradnelson@google.com, piman@chromium.org

Review URL: https://codereview.chromium.org/26621002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227752 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227752