Restrict __qem usage to UA sheets

LayoutTests/css3/supports-dom-api.html

Index: LayoutTests/css3/supports-dom-api.html
diff --git a/LayoutTests/css3/supports-dom-api.html b/LayoutTests/css3/supports-dom-api.html
index 07cc62fe36824e8999840f26eeda978d5aa8b9a0..8ac63b83f121a518381821838b0d3333f206bde6 100644
--- a/LayoutTests/css3/supports-dom-api.html
+++ b/LayoutTests/css3/supports-dom-api.html
@@ -84,6 +84,9 @@
     shouldBeFalse('CSS.supports("content", "!important")');
     shouldBeFalse('CSS.supports("(display: none)", undefined)');
 
+    // Test that __qem is internal
+    shouldBeFalse('CSS.supports("margin", "1__qem")');
+
     // shorthands and CSS wide keywords
     shouldBeTrue('CSS.supports("border", "1px solid #000")');
     shouldBeTrue('CSS.supports("border", "inherit")');