CrOS: Remove unused overflow button import from a browser test

Bug: 1020720
Change-Id: I32145a0c5cac946c461d3b7dbd842ccc7c60f799
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2089858Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747294}

chrome/browser/ui/ash/launcher/chrome_launcher_controller_browsertest.cc

@@ -16,7 +16,6 @@
 #include "ash/public/cpp/tablet_mode.h"
 #include "ash/public/cpp/window_properties.h"
 #include "ash/root_window_controller.h"
-#include "ash/shelf/overflow_button.h"
 #include "ash/shelf/shelf.h"
 #include "ash/shelf/shelf_app_button.h"
 #include "ash/shelf/shelf_layout_manager.h"