Backlight OCR: Apply Backlight's internal scale factor

Backlight currently uses an internal scale factor (1.25) to the values
in `viewport` passed to `ViewportUpdated()`. On the other hand,
`page_bitmap` received via `RequestBitmap()` has the image resolution
that's already applied with this internal scale factor. Thus, bounding
boxes for text extracted by OCR in the image contain values that are
applied with the internal scale factor. It leads to a discrepancy when
we do some computation using the bounding boxes and the viewport. To fix
the discrepancy, apply the internal scale factor.

navigating OCRed content in PDFs in Backlight.

AX-Relnotes: ChromeVox users will see correct bounding boxes when
Bug: 342563905
Change-Id: If4a06f52cd4803afd605de12bd28f0a96ccb9b7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5593597
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Kyungjun Lee <kyungjunlee@google.com>
Cr-Commit-Position: refs/heads/main@{#1310684}