Bug 836329:  Fix regression handling 'rem' units in media queries.  r=bzbarsky  approval-mozilla-aurora=akeybl

I confirmed that the added tests fail without the patch and pass with
the patch.

--HG--
extra : transplant_source : %28%E6%C1%11%25%83%1D6Vrc%11a%21%D0%AE%87h%C2%A1