Add drop down checkbox

This cl added a new component which is a label button with drop
down list of checkboxes. The data model is a ListModel with
std::u16string type and the selections are managed by a
ListSelectionModel.

Tested in style viewer.

Bug: b:308206058
Change-Id: Iece1957ec8c02788d5ac9ea61b70ce1cc6166a76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4982348
Reviewed-by: Yulun Wu <yulunwu@chromium.org>
Commit-Queue: Xiaodan Zhu <zxdan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1217172}

ash/BUILD.gn

@@ -1207,6 +1207,8 @@ component("ash") {
     "style/dark_light_mode_controller_impl.h",
     "style/dot_indicator.cc",
     "style/dot_indicator.h",
+    "style/drop_down_checkbox.cc",
+    "style/drop_down_checkbox.h",
     "style/harmonized_colors.cc",
     "style/harmonized_colors.h",
     "style/icon_button.cc",