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Previously if a host list fetch is not triggered by the user (refresh
control) and fails, then the app will simply pop up a toast complaining
about unavailable network and show nothing else. Pull-to-refresh
doesn't work.

This CL fixes this by providing a dialog-like view to allow user to
retry in that case.

Screenshot:
https://drive.google.com/file/d/0BytzIZKeM8nBMmN3b3A0bUhoMFU/view?usp=sharing

Bug: 771532
Change-Id: I64cb4015acc066322c3c6127b79b87d13cb06652
Reviewed-on: https://chromium-review.googlesource.com/700316Reviewed-by: Scott Nichols <nicholss@chromium.org>
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506537}