Restore private empty constructors in two download/home/filter files

This CL restores private empty constructors for helper classes
that only have static function or members.

These constructors were deleted in:
https://chromium-review.googlesource.com/c/chromium/src/+/1133382

Change-Id: Ie14035912e430e7b9973365ecc12bd78fab28af6
Reviewed-on: https://chromium-review.googlesource.com/1142406Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Marcin Wiącek <marcin@mwiacek.com>
Cr-Commit-Position: refs/heads/master@{#576501}

chrome/android/java/src/org/chromium/chrome/browser/download/home/filter/FilterModel.java

@@ -19,6 +19,8 @@ class FilterModel extends PropertyObservable<FilterModel.PropertyKey> {
         static final PropertyKey CONTENT_VIEW = new PropertyKey();
         static final PropertyKey SELECTED_TAB = new PropertyKey();
         static final PropertyKey CHANGE_LISTENER = new PropertyKey();
+
+        private PropertyKey() {}
     }
 
     private View mContentView;

chrome/android/java/src/org/chromium/chrome/browser/download/home/filter/Filters.java

@@ -104,4 +104,6 @@ public class Filters {
 
         return filter;
     }
+
+    private Filters() {}
 }
\ No newline at end of file