Handle backpress in the download manager

Bug: 859125
Change-Id: Ib8e85897d5059ec421ed9ddc63921a6005b37c9d
Reviewed-on: https://chromium-review.googlesource.com/1133175Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Peter Conn <peconn@chromium.org>
Commit-Queue: Anna Malova <amalova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574203}

chrome/android/java/src/org/chromium/chrome/browser/download/ui/DownloadManagerUi.java

@@ -327,11 +327,7 @@ public class DownloadManagerUi implements OnMenuItemClickListener, SearchDelegat
      */
     @Override
     public boolean onBackPressed() {
-        if (mBackendProvider.getSelectionDelegate().isSelectionEnabled()) {
-            mBackendProvider.getSelectionDelegate().clearSelection();
-            return true;
-        }
-        return false;
+        return mSelectableListLayout.onBackPressed();
     }
 
     /**