Disable some mojo_js_unittests on Mac and Linux.

BUG=351214
TBR=darin@chromium.org

Review URL: https://codereview.chromium.org/194473002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256151 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256151