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* Add fade in/out animation for ambient mode
* When showing ambient mode, fixed the first black screen issue
* When idling on lock screen and ambient mode returns, fixed the
  first image quickly flashes problem

This CL enables the default window animations, it also creates the
window only when images are successfully downloaded.

Bug: None
Change-Id: Ideb5683aefe92b5f375f06515db822bfd3cb7678
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2321714Reviewed-by: Meilin Wang <meilinw@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792432}