Add property change observer to checked_ property of CheckBox

Bug: 957832
Change-Id: Ieaf37b911a29bdf6462cf9a3e4faf81578a2d5f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2086558
Commit-Queue: Dan S <danan@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746947}

ui/views/controls/button/checkbox.cc

@@ -80,6 +80,11 @@ bool Checkbox::GetChecked() const {
   return checked_;
 }
 
+PropertyChangedSubscription Checkbox::AddCheckedChangedCallback(
+    PropertyChangedCallback callback) {
+  return AddPropertyChangedCallback(&checked_, callback);
+}
+
 void Checkbox::SetMultiLine(bool multi_line) {
   if (GetMultiLine() == multi_line)
     return;

ui/views/controls/button/checkbox.h

@@ -35,6 +35,9 @@ class VIEWS_EXPORT Checkbox : public LabelButton {
   virtual void SetChecked(bool checked);
   bool GetChecked() const;
 
+  PropertyChangedSubscription AddCheckedChangedCallback(
+      PropertyChangedCallback callback) WARN_UNUSED_RESULT;
+
   void SetMultiLine(bool multi_line);
   bool GetMultiLine() const;