Bug 1339301 - Downgrade unimplemented CSS rule from error to warning. r=heycam

MozReview-Commit-ID: 7brKkeCrbhZ

--HG--
extra : rebase_source : d4f0888673c94e593af53e2ad08a1f63efe425b0

layout/style/ServoCSSRuleList.cpp

@@ -71,7 +71,7 @@ ServoCSSRuleList::GetRule(uint32_t aIndex)
       case nsIDOMCSSRule::NAMESPACE_RULE:
         // XXX create corresponding rules
       default:
-        NS_ERROR("stylo: not implemented yet");
+        NS_WARNING("stylo: not implemented yet");
         return nullptr;
     }
     ruleObj->SetStyleSheet(mStyleSheet);