Bug 1388881 - Use namespace id instead of atom in synthesize_presentational_hints_for_legacy_attributes. r=manishearth

Using namespace id fixes this issue because in Gecko, the pref of MathML
(as well as SVG) works in the way that we choose a different namespace
id (the disabled id) for the elements. Those ids are mapped to the same
namespace atom as normal ids, which means if we use the atom, we would
treat the elements like normal mathml elements.

MozReview-Commit-ID: 9YBBokbP04M

--HG--
extra : rebase_source : 397f09db41a22bfa34e4abe26ad10027dab83d0d

layout/reftests/mathml/reftest.list

@@ -11,8 +11,8 @@ fails-if(!styloVsGecko) == dir-9.html dir-9-ref.html # Bug 787215
 == dir-10.html dir-10-ref.html
 == dir-11.html dir-11-ref.html
 == css-spacing-1.html css-spacing-1-ref.html
-pref(mathml.disabled,true) fails-if(styloVsGecko||stylo) == disabled-scriptlevel-1.html disabled-scriptlevel-1-ref.html
-pref(mathml.disabled,true) fails-if(styloVsGecko||stylo) == disabled-scriptlevel-1.xhtml disabled-scriptlevel-1-ref.xhtml
+pref(mathml.disabled,true) == disabled-scriptlevel-1.html disabled-scriptlevel-1-ref.html
+pref(mathml.disabled,true) == disabled-scriptlevel-1.xhtml disabled-scriptlevel-1-ref.xhtml
 == displaystyle-1.html displaystyle-1-ref.html
 == displaystyle-2.html displaystyle-2-ref.html
 == displaystyle-3.html displaystyle-3-ref.html