Separate client surface reference tracking from FrameGenerator.

services/ui/ws/BUILD.gn

Index: services/ui/ws/BUILD.gn
diff --git a/services/ui/ws/BUILD.gn b/services/ui/ws/BUILD.gn
index d4f5f07a7047b74412d5d97b2c35d20b4e34dc01..06c12719c055b08daaf4a82bff80bdae518a21aa 100644
--- a/services/ui/ws/BUILD.gn
+++ b/services/ui/ws/BUILD.gn
@@ -28,6 +28,8 @@ static_library("lib") {
     "drag_cursor_updater.h",
     "drag_source.h",
     "drag_target_connection.h",
+    "embedded_surface_tracker.cc",
+    "embedded_surface_tracker.h",
     "event_dispatcher.cc",
     "event_dispatcher.h",
     "event_dispatcher_delegate.h",
@@ -209,6 +211,7 @@ test("mus_ws_unittests") {
     "cursor_unittest.cc",
     "display_unittest.cc",
     "drag_controller_unittest.cc",
+    "embedded_surface_tracker_unittest.cc",
     "event_dispatcher_unittest.cc",
     "event_matcher_unittest.cc",
     "focus_controller_unittest.cc",
@@ -249,6 +252,7 @@ test("mus_ws_unittests") {
     "//services/ui/public/cpp",
     "//services/ui/public/cpp:internal_or_test",
     "//services/ui/public/interfaces",
+    "//testing/gmock",
     "//testing/gtest",
     "//third_party/mesa:osmesa",
     "//ui/aura",