Explicitly prevent URL-to-app redirection for forced downloads

There are 2 parts:
1) Replace ResourceRequestInfoImpl::is_download() with virtual ResourceRequestInfo::IsDownload().
2) Use it in app_url_redirector.cpp to explicitly avoid redirection of forced downloads.

@darin: In my testing, forced downloads were fine even without this (apparently, resource throttling is disabled in this case). Let me know if you still think explicit prevention is a good idea (I do think so).

BUG=302904

Review URL: https://codereview.chromium.org/62463004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237570 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237570