Issue #1619 - Convert Intrinsic Ratio to Float

https://bugzilla.mozilla.org/show_bug.cgi?id=1547792

Aspect Ratio handling simplified by using floating point integers:
- Multiplication of value (or inverse value) to a known side for Scaling
- No unequal equal values such as "4/3" vs "8/6" vs "20/15"
- Truly "Empty" aspect ratios, even if one dimension is not 0

layout/style/nsStyleStruct.cpp

@@ -2731,7 +2731,7 @@ nsStyleImageLayers::Size::DependsOnPositioningAreaSize(const nsStyleImage& aImag
     }
     if (imgContainer) {
       CSSIntSize imageSize;
-      nsSize imageRatio;
+      AspectRatio imageRatio;
       bool hasWidth, hasHeight;
       nsLayoutUtils::ComputeSizeForDrawing(imgContainer, imageSize, imageRatio,
                                            hasWidth, hasHeight);
@@ -2744,7 +2744,7 @@ nsStyleImageLayers::Size::DependsOnPositioningAreaSize(const nsStyleImage& aImag
 
       // If the image has an intrinsic ratio, rendering will depend on frame
       // size when background-size is all auto.
-      if (imageRatio != nsSize(0, 0)) {
+      if (imageRatio) {
         return mWidthType == mHeightType;
       }