Bug 1291390 - Use the parsing mode from the stylesheet when building preferences stylesheet. r=heycam

In practice this will be the 'agent' level rather than the 'author' level, given
the SetParsingMode call immediately after creating the stylesheet. This change
aligns behavior with CSSStyleSheet::ReparseSheet.

layout/style/StyleSheet.h

@@ -43,6 +43,7 @@ public:
     mParsingMode = aParsingMode;
   }
 
+  css::SheetParsingMode ParsingMode() { return mParsingMode; }
   mozilla::dom::CSSStyleSheetParsingMode ParsingModeDOM();
 
   nsINode* GetOwnerNode() const { return mOwningNode; }

layout/style/nsLayoutStylesheetCache.cpp

@@ -951,8 +951,7 @@ nsLayoutStylesheetCache::BuildPreferenceSheet(StyleSheetHandle::RefPtr* aSheet,
   if (sheet->IsGecko()) {
     sheet->AsGecko()->ReparseSheet(sheetText);
   } else {
-    sheet->AsServo()->ParseSheet(sheetText, uri, uri, nullptr, 0,
-                                 SheetParsingMode::eAuthorSheetFeatures);
+    sheet->AsServo()->ParseSheet(sheetText, uri, uri, nullptr, 0, sheet->AsServo()->ParsingMode());
   }
 
 #undef NS_GET_R_G_B