Add multiple selection support to chooser on desktops

This CL is part 1 of changing Chrome Apps device permissions dialog
using the same chooser dialog as WebUSB and WebBluetooth. Since the
device permissions dialog allows multiple selection, this CL added code
to support multiple selection for chooser.

BUG=653222

Review-Url: https://codereview.chromium.org/2518933004
Cr-Commit-Position: refs/heads/master@{#434209}
diff --git a/chrome/browser/chooser_controller/chooser_controller.cc b/chrome/browser/chooser_controller/chooser_controller.cc
index af4c28f..6bca41ef 100644
--- a/chrome/browser/chooser_controller/chooser_controller.cc
+++ b/chrome/browser/chooser_controller/chooser_controller.cc
@@ -56,6 +56,10 @@
   return false;
 }
 
+bool ChooserController::AllowMultipleSelection() const {
+  return false;
+}
+
 int ChooserController::GetSignalStrengthLevel(size_t index) const {
   return -1;
 }