Bug 1835447. Use print pages' own computed size as their container size. r=emilio

  const WritingMode wm = aReflowInput.GetWritingMode();

  // This is the app-unit size of each page (in physical & logical units):

  const nsSize physPageSize = aPresContext->GetPageSize();

  const LogicalSize pageSize(wm, physPageSize);

  // Count the number of pages that are displayed on this sheet (i.e. how many

  // child frames we end up laying out, excluding any pages that are skipped

  // due to not being in the user's page-range selection).

      numPagesOnThisSheet++;

    }

    // This is the app-unit size of the page (in physical & logical units).

    // Note: The page sizes come from CSS or else from the user selected size;

    // pages are never reflowed to fit their sheet - if/when necessary they are

    // scaled to fit their sheet. Hence why we get the page's own dimensions to

    // use as its "available space"/"container size" here.

    const nsSize physPageSize = pageFrame->ComputePageSize();

    const LogicalSize pageSize(wm, physPageSize);

    ReflowInput pageReflowInput(aPresContext, aReflowInput, pageFrame,

                                pageSize);