Simplify calculation in PDFiumEngine::GetRegion()

This method applies the same offset to a point and a rect before
determining if the point is in the rect. The same result can be achieved
by not applying the offset at all.

Change-Id: I05a313ed93e52d4b6a279f2f078136e43406240c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5617122


Reviewed-by: Alan Screen <awscreen@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313153}