[PDF] Add wrapper for FPDFAnnot_GetRect()

Add a C++ wrapper to return an optional rect instead of returning a bool
and using an out-parameter. To make this work without copying, change
PdfRect so it has the same structure as PDFium's FS_RECTF. Then also add
FsRectFFromPdfRect() conversion functions to make casting to FS_RECTF
more readable.

Change-Id: Ie0df6f0a04c243860b3357c45b8562cbf83e1446
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6854211
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Andy Phan <andyphan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1502244}