bug 804644 - convert Cocoa points to _un_scaled device pixels for system font sizes. r=roc

layout/style/nsRuleNode.cpp

@@ -3003,8 +3003,9 @@ nsRuleNode::SetFont(nsPresContext* aPresContext, nsStyleContext* aContext,
     gfxFontStyle fontStyle;
     LookAndFeel::FontID fontID =
       (LookAndFeel::FontID)systemFontValue->GetIntValue();
-    float devPerCSS = (float)nsPresContext::AppUnitsPerCSSPixel() /
-                      aPresContext->AppUnitsPerDevPixel();
+    float devPerCSS =
+      (float)nsPresContext::AppUnitsPerCSSPixel() /
+      aPresContext->DeviceContext()->UnscaledAppUnitsPerDevPixel();
     if (LookAndFeel::GetFont(fontID, systemFont.name, fontStyle, devPerCSS)) {
       systemFont.style = fontStyle.style;
       systemFont.systemFont = fontStyle.systemFont;