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With the new profiles UI and the high res avatar downloading code, we are sending out
waaaay too many OnProfileAvatarChanged notifications. This is particularly terrible on
Windows, where basically on every Chrome startup we refresh all the icons.

Most importantly, we need to stop downloading the avatar files every time profiles get
loaded in the ProfileInfoCache. We did this originally because the avatar files were pretty
volatile, but it's safe to assume they've stopped changing.

Secondly, the only things that care about the high res avatar files are the UserManager
and the profile switcher, so they should get a special notification that this particular
file is ready, rather that annoy all the other listeners who only care about the tiny avatars.

BUG=449569

Review URL: https://codereview.chromium.org/861053004

Cr-Commit-Position: refs/heads/master@{#313593}