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This CL implements a loading view (aka progress bar) in download manager
UI on Android. It is slightly trickier to handle state changes on
Tablets, because when the download ui is initialized, a filter is
already set in the URL. Therefore we cache the filter temporarily in the
manager, and will show it after we load everything.

There are three possible types of backend the UI needs to wait:
incognito downloads, downloads and offline apges. The UI will only claim
it is loaded if all of them are loaed.

TODO(ianwen): add a timeout mechanisim to either the
DownloadLoadingDelegate or to the backend so that if it takes forever to
load one of the backend, users are still able to see the other two.

BUG=616324

Review-Url: https://codereview.chromium.org/2269353004
Cr-Commit-Position: refs/heads/master@{#414369}