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This cl moves network icon cache purging to a new observer that is owned
by the unified system tray.

Currently there are five subclasses of
TrayNetworkStateObserver::Delegate. Every time we get a network list
update the global network icon cache is purged.
This means that for every update the cache is purged at least five
times.

With this cl TrayNetworkStateObserver::Delegate can still be used as a
simple way of subscribing to all network events without excessive icon
purging.

Bug: 902409
Change-Id: I078ac8dbdee473fc285ee8e91f735c99524c74a7
Reviewed-on: https://chromium-review.googlesource.com/c/1334835
Commit-Queue: Tony De Luna <tonydeluna@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608230}