[PDF] Clean up PDFiumPage::GetCharIndex()

- Rename it to GetCharInfo(), since it does a lot more than the name
  implies.
- Remove `rv` variable.
- Clang-format the method.
- Rewrite the documentation to be clearer.

Change-Id: I9212372a5d9b6d7c320890b0931e93636f04ab7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6926884
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Andy Phan <andyphan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1513370}