Bug 1246346 - patch 1 - Expose a DesktopToDevice scale factor on nsDeviceContext. r=emk

gfx/src/nsDeviceContext.cpp

@@ -722,3 +722,18 @@ nsDeviceContext::UpdateAppUnitsForFullZoom()
     // adjust mFullZoom to reflect appunit rounding
     mFullZoom = float(mAppUnitsPerDevPixelAtUnitFullZoom) / mAppUnitsPerDevPixel;
 }
+
+DesktopToLayoutDeviceScale
+nsDeviceContext::GetDesktopToDeviceScale()
+{
+    nsCOMPtr<nsIScreen> screen;
+    FindScreen(getter_AddRefs(screen));
+
+    if (screen) {
+        double scale;
+        screen->GetContentsScaleFactor(&scale);
+        return DesktopToLayoutDeviceScale(scale);
+    }
+
+    return DesktopToLayoutDeviceScale(1.0);
+}

gfx/src/nsDeviceContext.h

@@ -251,6 +251,8 @@ public:
      */
     bool IsPrinterSurface();
 
+    mozilla::DesktopToLayoutDeviceScale GetDesktopToDeviceScale();
+
 private:
     // Private destructor, to discourage deletion outside of Release():
     ~nsDeviceContext();