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This adds a delegate method to ContentSuggestionsHeaderViewControllerDelegate
protocol specify how user's avatar image would be processed.

New Tab Page uses this delegate method to add/update the avatar for the
current user's identity and display at the top right corner of NTP.

Bug: 961120
Change-Id: I19abd5eab02481214a605de4b6321aa9a22ea79f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1612091Reviewed-by: Peter Lee <pkl@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660608}