Bug 1205511 - Give nsPresContext a method that returns the CSS to device pixel scale. r=dbaron

--HG--
extra : source : 50b0470193cef87e4292aee14dc940b39864832d

layout/base/nsPresContext.h

@@ -38,6 +38,7 @@
 #include "ScrollbarStyles.h"
 #include "nsIMessageManager.h"
 #include "mozilla/RestyleLogging.h"
+#include "Units.h"
 
 class nsAString;
 class nsIPrintSettings;
@@ -636,6 +637,12 @@ public:
   float DevPixelsToFloatCSSPixels(int32_t aPixels)
   { return AppUnitsToFloatCSSPixels(DevPixelsToAppUnits(aPixels)); }
 
+  mozilla::CSSToLayoutDeviceScale CSSToDevPixelScale() const
+  {
+    return mozilla::CSSToLayoutDeviceScale(
+        float(AppUnitsPerCSSPixel()) / float(AppUnitsPerDevPixel()));
+  }
+
   // If there is a remainder, it is rounded to nearest app units.
   nscoord GfxUnitsToAppUnits(gfxFloat aGfxUnits) const;