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The generic icon should be shown when there's only one (non-signed in)
user. We need to listen for add/remove profile events so we can detect
whether a generic icon should be shown or not for existing windows as
well.

Bug: chromium:822070
Change-Id: Ia84feb13dd10b4459aeff326e2fd6e1d29698a76
Reviewed-on: https://chromium-review.googlesource.com/1028981Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553912}