Add multiple selection support to chooser on desktops

chrome/browser/chooser_controller/chooser_controller.h

Index: chrome/browser/chooser_controller/chooser_controller.h
diff --git a/chrome/browser/chooser_controller/chooser_controller.h b/chrome/browser/chooser_controller/chooser_controller.h
index 5faceec34bec860d6bcb273b7389fe8b3581ca45..48ed7a82ad0bdb0fa2cf087b14f5a31d8bf2378d 100644
--- a/chrome/browser/chooser_controller/chooser_controller.h
+++ b/chrome/browser/chooser_controller/chooser_controller.h
@@ -66,6 +66,9 @@ class ChooserController {
   // For WebBluetooth, it is a signal strength icon.
   virtual bool ShouldShowIconBeforeText() const;
 
+  // Returns if the chooser allows multiple items to be selected.
+  virtual bool AllowMultipleSelection() const;
+
   // Returns the text to be displayed in the chooser when there are no options.
   virtual base::string16 GetNoOptionsText() const = 0;