Bug 1732600. Make GetDisplayPortFromMarginsData use ancestor process transform to ancestor scale. r=botond

Differential Revision: https://phabricator.services.mozilla.com/D126661

layout/base/DisplayPortUtils.cpp

@@ -295,9 +295,11 @@ static nsRect GetDisplayPortFromMarginsData(
   nsPresContext* presContext = frame->PresContext();
   int32_t auPerDevPixel = presContext->AppUnitsPerDevPixel();
 
-  LayoutDeviceToScreenScale2D res(
-      presContext->PresShell()->GetCumulativeResolution() *
-      nsLayoutUtils::GetTransformToAncestorScale(frame));
+  LayoutDeviceToScreenScale2D res =
+      LayoutDeviceToParentLayerScale(
+          presContext->PresShell()->GetCumulativeResolution()) *
+      nsLayoutUtils::GetTransformToAncestorScaleCrossProcessForFrameMetrics(
+          frame);
 
   // Calculate the expanded scrollable rect, which we'll be clamping the
   // displayport to.