Added UMA statistics for changing the active window via click or touch events.

ui/wm/BUILD.gn

Index: ui/wm/BUILD.gn
diff --git a/ui/wm/BUILD.gn b/ui/wm/BUILD.gn
index d149b48d08b30fcc889d12f1e796ae7663a33516..be5bc72c44d2ead19616d694d34fe50b297b194f 100644
--- a/ui/wm/BUILD.gn
+++ b/ui/wm/BUILD.gn
@@ -59,6 +59,10 @@ component("wm") {
     "core/transient_window_stacking_client.h",
     "core/visibility_controller.cc",
     "core/visibility_controller.h",
+    "core/window_activation_pre_target_handler.cc",
+    "core/window_activation_pre_target_handler.h",
+    "core/window_activation_pre_target_handler_observer.cc",
+    "core/window_activation_pre_target_handler_observer.h",
     "core/window_animations.cc",
     "core/window_animations.h",
     "core/window_modality_controller.cc",