Bug 1675852 Part 2 - Define extra fallback intrinsic sizes in app units. r=emilio

CSSPixel::ToAppUnits() requires computation at runtime. Since most of
the layout computations require `nscoord`, this patch defines extra
intrinsic sizes in app units.

Differential Revision: https://phabricator.services.mozilla.com/D96258

layout/base/LayoutConstants.h

@@ -90,6 +90,13 @@ inline constexpr CSSIntCoord kFallbackIntrinsicHeightInPixels(150);
 inline constexpr CSSIntSize kFallbackIntrinsicSizeInPixels(
     kFallbackIntrinsicWidthInPixels, kFallbackIntrinsicHeightInPixels);
 
+inline constexpr nscoord kFallbackIntrinsicWidth =
+    kFallbackIntrinsicWidthInPixels * AppUnitsPerCSSPixel();
+inline constexpr nscoord kFallbackIntrinsicHeight =
+    kFallbackIntrinsicHeightInPixels * AppUnitsPerCSSPixel();
+inline constexpr nsSize kFallbackIntrinsicSize(kFallbackIntrinsicWidth,
+                                               kFallbackIntrinsicHeight);
+
 /**
  * This is used in some nsLayoutUtils functions.
  * Declared here so that fewer files need to include nsLayoutUtils.h.

layout/generic/nsSubDocumentFrame.cpp

@@ -635,7 +635,7 @@ IntrinsicSize nsSubDocumentFrame::GetIntrinsicSize() {
   }
 
   // We must be an HTML <iframe>. Return fallback size.
-  return IntrinsicSize(CSSPixel::ToAppUnits(kFallbackIntrinsicSizeInPixels));
+  return IntrinsicSize(kFallbackIntrinsicSize);
 }
 
 /* virtual */

layout/generic/nsVideoFrame.cpp

@@ -684,7 +684,7 @@ nsSize nsVideoFrame::GetVideoIntrinsicSize() const {
     }
   }
 
-  return CSSPixel::ToAppUnits(kFallbackIntrinsicSizeInPixels);
+  return kFallbackIntrinsicSize;
 }
 
 IntrinsicSize nsVideoFrame::GetIntrinsicSize() {