Use DownloadManager::Observer in TestDownloadShelf

Use DownloadManager::Observer interface to track the lifetime of
DownloadManager instead of keeping a counted reference to it.

This will easy the transition of DownloadManager from being refcounted
to be owned by the BrowserContext.

TEST=unit_tests with filter for Download\*

Review URL:

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