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This is a follow-up to https://codereview.chromium.org/874283006
which introduces the cr-network-icon Polymer element for
displaying an icon representing network state in web UI.

It also includes minor changes to the fake shill implementation to facilitate manual testing.

BUG=455499

Review URL: https://codereview.chromium.org/954293003

Cr-Commit-Position: refs/heads/master@{#318824}