[PDF Ink Signatures] Use more realistic page/viewport dimensions in test

This is in preparation for clamping the textbox to the page area.
90x90 or 80x100 pages with a window size of just 100x100 are not
realistic and are too small to fit a textbox with any significant
amount of text. They also don't accommodate the default text box size
from mocks.

Use the 400x500 size already used by some of the manager tests for all
tests that rely on the mock viewport and adjust test expectations
accordingly.

Bug: 402546155
Change-Id: I6365e6d73c0a11863da3c164b57f8810a59dda92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6579846
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Andy Phan <andyphan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1466187}