[Android] Make ToolbarPhone.mMenuButton private

This CL makes ToolbarPhone.mMenuButton private without any changes to
the behavior.

Bug: None
Change-Id: I267a9926dfa63614168ac7ada2a35629591222c0
Reviewed-on: https://chromium-review.googlesource.com/c/1323671Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606165}

chrome/android/java/src/org/chromium/chrome/browser/toolbar/ToolbarLayout.java

@@ -61,7 +61,7 @@ public abstract class ToolbarLayout extends FrameLayout implements Toolbar {
     /**
      * The app menu button.
      */
-    protected ImageButton mMenuButton;
+    private ImageButton mMenuButton;
     private ImageView mMenuBadge;
     private MenuButton mMenuButtonWrapper;
     private AppMenuButtonHelper mAppMenuButtonHelper;

chrome/android/java/src/org/chromium/chrome/browser/toolbar/ToolbarPhone.java

@@ -2727,7 +2727,7 @@ public class ToolbarPhone extends ToolbarLayout
      * buttons besides the experimental button.
      */
     private boolean isMenuButtonPresent() {
-        return mMenuButton != null;
+        return getMenuButton() != null;
     }
 
     private void requestLayoutHostUpdateForExperimentalButton() {