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Discard thumbnail image data for thumbnails that are far from view to
save memory.

The top margin of 500% was selected with the assumption that every
100% height of the thumbnail bar can fit ~4 thumbnails on an average
screen. Along with the 100% bottom margin and the visible area, around
30 thumbnails worth of image data would be kept in memory at any given
time. Metrics also show that ~90% of PDFs are less than 30 pages.

Bug: 652400
Change-Id: I92126374ad715b502fd6e052246e537d65c407e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2439645Reviewed-by: dpapad <dpapad@chromium.org>
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812752}