[PDF Searchify] Avoid Searchifying the same page twice

Extend PDFiumOnDemandSearchifierTest.PageWithImagesNoRecognizableText to
perform a page unload followed by a reload. This triggers a CHECK()
failure that can be seen in crash reports in the wild. Fix this crash by
modifying the PDFiumEngine::ScheduleSearchifyIfNeeded() logic to not
schedule Searchify for pages that have been Searchified, regardless of
whether Searchify added text to the page or not.

Bug: 395485272
Change-Id: I58772f8f2c71d271c61898b6b3364c4df089027d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6250975
Reviewed-by: Ramin Halavati <rhalavati@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1419360}