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This is caused by racing between mojo and non-mojo code path.
The avater image is updated via SessionController mojo interface
but UserObserver interface is still a direct call and could reach
ash before the mojo message. The CL fixes the issue by replacing
UserObserver interface with SessionController observer.

BUG=704862

Review-Url: https://codereview.chromium.org/2780963002
Cr-Commit-Position: refs/heads/master@{#460571}