Use a real download item.

BUG=14659,17100
TEST=Download multiple things. Items should now be inserted from the left. When a download is in progress, the time to completion should be displayed, when it's done the time should fade out and the filename should move down. Download items should appear in a sweep animation. Clicking the download directly should open it; clicking the arrow on the right of the download item should show the correct context menu. If a download filename is long, it should be elided in the middle.

Review URL: http://codereview.chromium.org/159060

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@21201 0039d316-1c4b-4281-b951-d872f2087c98