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The only remaining usage is in tests. Convert the test to
ContextLossObserver instead and delete ContextFactoryObserver.

Bug: 947279
Change-Id: I0ec87d11a011ea315dc1a4cea3b8bf66f1f30f91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1546269Reviewed-by: Saman Sami <samans@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735086}