Fix nits in PdfOcrService and related tests

This is a follow-up to https://crrev.com/1190258 to do the following
refactoring changes:

- Rename ResetPageCount() to SetPageCount() and update the comments that
  explain what it does.
- Change SetPageCount() to also update `pages_per_batch_`, so the caller
  can just make one call per page count update.
- Stop initializing `pages_per_batch_` in the header, as the ctor is
  doing it instead. Also move this member variable to the correct place
  in the header.
- Change ComputeAndSetPagesPerBatch() to ComputePagesPerBatch() to make
  its use more flexible.
- Replace std::pair in tests with a struct.
- Mark several getter methods as const.

Change-Id: I72f156cdbdd9461892da0f1f4ffd89a2c4bee2c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4846813
Reviewed-by: Kyungjun Lee <kyungjunlee@google.com>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1193320}