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We don't want portals to be an avenue for download spam,
and we aren't presently aware of uses that require
downloads in portals.

This is largely straightforward, but the non-chrome
DownloadManagerDelegate was unfortunately left as a stub
that never calls WebContentsDelegate::CanDownload (like
the chrome/ one does).

Accordingly, this CL expands it to a simplified version
of what the chrome/ one does. The post-task was left
because the TODO comment there insinuates that not doing
so is risky. It would be straightforward to inline the
posted task in the future if one wished to.

This strategy is used instead of a NavigationThrottle,
because NavigationThrottle only covers downloads that are
initiated by navigation, but <a download> and other
renderer-initiated downloads do not fire navigation
throttles.

AwWebContentsDelegate::CanDownload is removed because it
was previously unreachable but is now reachable (because
AwDownloadManagerDelegate does not override
CheckDownloadAllowed), but behavior is otherwise preserved.
Android WebView relies on
AwDownloadManagerDelegate::InterceptDownloadIfApplicable
for its download interception features.

Bug: 967202
Change-Id: Ic019688aab81ebc92728bfd715bb10a5de3f36d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2229463Reviewed-by: Lucas Gadani <lfg@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775843}