[PDF Ink Signatures] Fix getPageScreenRect for 2-up view

getPageScreenRect() did not return the correct screen rectangle in 2-up
view. This CL fixes the method to return the correct rectangle for each
page regardless of the view.

This is in preparation for restricting text annotation boxes to a page.
Restricting the box the the page requires having an accurate screen
rectangle for the page.

Bug: 402546155
Change-Id: I01950ec9d5b51a33190f7d3913770262baf3dddc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6598937
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Andy Phan <andyphan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1466777}