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This is a replacement for CaptureThumbnailOnLoadFinished, which was
reverted due to perf regressions (see
https://chromium-review.googlesource.com/c/570218/).

Bug: 718413
Change-Id: I399507f5791b5ca1fc7779331b82bb9782a27268
Reviewed-on: https://chromium-review.googlesource.com/571223Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487424}