Change CaptureSynchronizer and PointerWatcherEventRouter to support multiple CaptureClients.

ui/aura/mus/window_tree_client.cc

Index: ui/aura/mus/window_tree_client.cc
diff --git a/ui/aura/mus/window_tree_client.cc b/ui/aura/mus/window_tree_client.cc
index 13660c96943379fbde45e8063fb00f656942a44e..78890be25da267c2e3e0cd9ddf4606e02dbfe49b 100644
--- a/ui/aura/mus/window_tree_client.cc
+++ b/ui/aura/mus/window_tree_client.cc
@@ -21,6 +21,7 @@
 #include "services/ui/public/interfaces/window_manager.mojom.h"
 #include "services/ui/public/interfaces/window_manager_window_tree_factory.mojom.h"
 #include "ui/aura/client/aura_constants.h"
+#include "ui/aura/client/capture_client.h"
 #include "ui/aura/client/drag_drop_client.h"
 #include "ui/aura/client/transient_window_client.h"
 #include "ui/aura/mus/capture_synchronizer.h"
@@ -453,8 +454,7 @@ void WindowTreeClient::WindowTreeConnectionEstablished(
   tree_ = window_tree;
 
   drag_drop_controller_ = base::MakeUnique<DragDropControllerMus>(this, tree_);
-  capture_synchronizer_ =
-      base::MakeUnique<CaptureSynchronizer>(this, tree_, GetCaptureClient());
+  capture_synchronizer_ = base::MakeUnique<CaptureSynchronizer>(this, tree_);
   focus_synchronizer_ = base::MakeUnique<FocusSynchronizer>(this, tree_);
 }
 
@@ -464,9 +464,15 @@ void WindowTreeClient::OnConnectionLost() {
 
 bool WindowTreeClient::HandleInternalPropertyChanged(WindowMus* window,
                                                      const void* key) {
-  if (key != client::kModalKey)
+  if (key != client::kModalKey && key != client::kRootWindowCaptureClientKey)
     return false;
 
+  if (key == client::kRootWindowCaptureClientKey) {
+    capture_synchronizer_->Attach(
+        client::GetCaptureClient(window->GetWindow()));
+    return true;
+  }
+
   if (window->GetWindow()->GetProperty(client::kModalKey) ==
       ui::MODAL_TYPE_NONE) {
     // TODO: shouldn't early return, but explicitly tell server to turn off