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Add or remove pages from the viewer in response to these events.

Bug: pdfium:401,chromium:872903,chromium:867135
Change-Id: Iddc88c5a90370213619a0f3f2587f215d0dafbc8
Reviewed-on: https://chromium-review.googlesource.com/1169691
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584515}