Fix download shelf visibility bug introduced by nullptr change.

The bug caused the download shelf visibility to not be updated
correctly.

BUG=434594

Review URL: https://codereview.chromium.org/740203002

Cr-Commit-Position: refs/heads/master@{#305118}

chrome/browser/ui/views/frame/browser_view.cc

@@ -1219,7 +1219,6 @@ void BrowserView::SetDownloadShelfVisible(bool visible) {
     GetDownloadShelf();
   }
 
-  if (!browser_)
   browser_->UpdateDownloadShelfVisibility(visible);
 
   // SetDownloadShelfVisible can force-close the shelf, so make sure we lay out