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This is a reland of 581c91ec

Original change's description:
> ambient: start when two images loaded
>
> Construct ambient widget when two images are in memory to prevent burn
> in.
>
> BUG=b:167332126
>
> Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
> Change-Id: I520ada00429125d35bda9ef66089ca9a9c8d4440
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2440306
> Commit-Queue: Jeffrey Young <cowmoo@chromium.org>
> Reviewed-by: Tao Wu <wutao@chromium.org>
> Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#814711}

Bug: b:167332126
Change-Id: I7fe0cca3ecb15262666eee1067f57bc21b150468
Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2459399Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Jeffrey Young <cowmoo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815206}