Create BrowserUserData.

ios/shared/chrome/browser/ui/browser_list/browser.h

Index: ios/shared/chrome/browser/ui/browser_list/browser.h
diff --git a/ios/shared/chrome/browser/ui/browser_list/browser.h b/ios/shared/chrome/browser/ui/browser_list/browser.h
index ef6cbb0002ac860c3bbcce78e08d0315be8768e4..213f8d1d4737bc1ac2a0db8c1abf94b46cade1e4 100644
--- a/ios/shared/chrome/browser/ui/browser_list/browser.h
+++ b/ios/shared/chrome/browser/ui/browser_list/browser.h
@@ -8,6 +8,7 @@
 #include <memory>
 
 #include "base/macros.h"
+#include "base/supports_user_data.h"
 
 class WebStateList;
 class WebStateListDelegate;
@@ -21,10 +22,10 @@ class ChromeBrowserState;
 
 // Browser holds the state backing a collection of Tabs and the attached
 // UI elements (Tab strip, ...).
-class Browser {
+class Browser : public base::SupportsUserData {
  public:
   explicit Browser(ios::ChromeBrowserState* browser_state);
-  ~Browser();
+  ~Browser() override;
 
   WebStateList& web_state_list() { return *web_state_list_.get(); }
   const WebStateList& web_state_list() const { return *web_state_list_.get(); }