Make the UI an observer of downloads.

This relieves content/browser/download from knowing who/when to notify regarding a new download.

BUG=147753


Review URL: https://chromiumcodereview.appspot.com/11640007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@186458 0039d316-1c4b-4281-b951-d872f2087c98
37 files changed