Reland "Extract high quality images from PDF only when they are needed for OCR."

This is a reland of commit 5208eafb234ba2af392851bc75e2dbac8ca288e2.
Fixed the build issue by moving the definition of
`TestPdfAccessibilityTree` out of BUILDFLAG(ENABLE_SCREEN_AI_SERVICE)
and wrapping only necessary class functions and members with the
buildflag.

Original change's description:
> Extract high quality images from PDF only when they are needed for OCR.
>
> 32-bit bitmaps with highest available quality are extracted from PDF
> files when they are loaded, so that they would be sent later to OCR
> service. To avoid the memory overhead of this process, this CL
> postpones image extraction to the time they are sent to OCR service,
> and destroys the extracted images immediately after that.
>
> AX-Relnotes: n/a
> Bug: 1471392
> Change-Id: Id337edf693d8d4a4ddd1a56d814a0d1f0e1ac5e4
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4754282
> Auto-Submit: Ramin Halavati <rhalavati@chromium.org>
> Reviewed-by: Kyungjun Lee <kyungjunlee@google.com>
> Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
> Commit-Queue: Lei Zhang <thestig@chromium.org>
> Reviewed-by: Lei Zhang <thestig@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1182463}

Bug: 1471392
Change-Id: I1762da4b5d5895ce112f62d0ca99c23236a6135d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4775300
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Kyungjun Lee <kyungjunlee@google.com>
Cr-Commit-Position: refs/heads/main@{#1182838}