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After 3 consecutive failures to load an image, notify observers of
failure.

BUG=b:169591750
TEST=ash_unittests --gtest_filter=AmbientBackendModelTest*

Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Change-Id: I7a9302e65fc58d3272a0da31f13c6a44efe1cc58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2449937
Commit-Queue: Jeffrey Young <cowmoo@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816733}