Make Items in DropdownPopupWindow always focusable

Make sure that AnchoredPopupWindow inside of DropdownPopupWindow
is always focusable, so the items in the list can be selected.

Bug: 820898
Change-Id: I8ede526787cad6bb585013d2c4280d502eff7652
Reviewed-on: https://chromium-review.googlesource.com/974172Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Amirhossein Simjour <asimjour@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545066}

ui/android/java/src/org/chromium/ui/DropdownPopupWindow.java

@@ -82,6 +82,7 @@ public class DropdownPopupWindow implements AnchoredPopupWindow.LayoutObserver {
         mHorizontalPadding = paddingRect.right + paddingRect.left;
         mAnchoredPopupWindow.setPreferredHorizontalOrientation(
                 AnchoredPopupWindow.HORIZONTAL_ORIENTATION_CENTER);
+        mAnchoredPopupWindow.setFocusable(true);
     }
 
     /**