[PDF Ink Signatures] Scroll the PDF to the caret on caret move

Scroll the PDF rendering to the caret when the caret is set or moved. To
do so, add a PdfCaretClient::ScrollToChar to scroll to a certain
character. In PDFiumEngine's implementation, the screen rect of the
character is used, but not all characters have screen rects. Since
PdfCaret already has an algorithm to find the previous character with a
screen rect, use the same algorithm to pass into ScrollToChar().

Bug: 445966256
Change-Id: Ice72a10bf2db7cfad5a873e163ac9e59bb5aac68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7009129
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Andy Phan <andyphan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1527158}