Issue #1620 - Use Intrinsic Aspect Ratio for Images

https://bugzilla.mozilla.org/show_bug.cgi?id=1547231
https://bugzilla.mozilla.org/show_bug.cgi?id=1559094
https://bugzilla.mozilla.org/show_bug.cgi?id=1633434
https://bugzilla.mozilla.org/show_bug.cgi?id=1565690
https://bugzilla.mozilla.org/show_bug.cgi?id=1602047

Make use of Aspect Ratios in Image frames before Images are loaded.
- Check for width and height HTML properties and create a ratio with them.
- Overwrite HTML size values with actual image dimensions on load.
- Collapse any frames with srcless images.

Comments:
dom/html/nsGenericHTMLElement.cpp:1483
layout/generic/nsImageFrame.cpp:289

dom/html/HTMLImageElement.h

@@ -196,6 +196,11 @@ public:
     return GetReferrerPolicyAsEnum();
   }
 
+  bool IsAwaitingLoad() const
+  {
+    return !!mPendingImageLoadTask;
+  }
+
   int32_t X();
   int32_t Y();
   // Uses XPCOM GetLowsrc.