DownloadTestObserver should know when the DownloadManager was shutdown

DownloadTestObserver now uses ManagerGoingDown() interface to know
whether its manager was shutdown. This allows it to use a raw pointer
instead of a refptr.

By removing the refptr to DownloadManager, this commit will help the
change of DownloadManager to be owned by BrowserContext instead of a
refcounted object.

Previously we were piggybacking on the refptr to bind callbacks with
download manager. Now we use weak pointers to the DownloadTestObserver
object itself, and it'll guard whether the download manager was
shutdown or not.

This patch also removes some outdated commentary about "file select

TEST=content_unittests and browser_tests with filter for Download\*

Review URL:

git-svn-id: svn:// 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed