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Previously we allow to prefetch any preset number of images, but this is
not required and increases the complexity of code.
This patch simplifies the photo refresh logic. We only prefetch one more
image than displayed, which is used for photo transition animation.

Bug: b/156271483
Test: new unittests.
Change-Id: I0222c8f4db80d98dbd8f65453b4f8fe07bcb59e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209538Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770850}