Fix profile-keyed dependencies for custodian profile downloader

The CustodianProfileDownloaderService depends on the
ProfileOAuth2TokenServiceFactory via a ProfileDownloader, so tell the
BrowserContextDependencyManager. Also clean up the ProfileDownloader during the
Shutdown phase rather than letting it sit until the service is destroyed. This
fixes a DCHECK encountered during shutdown when the ProfileDownloader tries to
access a BrowserContext that's been destroyed.

BUG=302436

Review URL: https://codereview.chromium.org/27098004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228660 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228660