Change CaptureSynchronizer and PointerWatcherEventRouter to support multiple CaptureClients.

ui/views/mus/desktop_window_tree_host_mus.cc

Index: ui/views/mus/desktop_window_tree_host_mus.cc
diff --git a/ui/views/mus/desktop_window_tree_host_mus.cc b/ui/views/mus/desktop_window_tree_host_mus.cc
index 02cf6376934d0bff1123c9b9149b382b160d6280..36448839c4944057dde9b15522dda0fd9d56d9d7 100644
--- a/ui/views/mus/desktop_window_tree_host_mus.cc
+++ b/ui/views/mus/desktop_window_tree_host_mus.cc
@@ -7,6 +7,7 @@
 #include "base/memory/ptr_util.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "ui/aura/client/aura_constants.h"
+#include "ui/aura/client/capture_client.h"
 #include "ui/aura/client/cursor_client.h"
 #include "ui/aura/client/drag_drop_client.h"
 #include "ui/aura/client/focus_client.h"
@@ -303,6 +304,9 @@ void DesktopWindowTreeHostMus::OnWidgetInitDone() {
   // client-area and hit-test-mask.
   SendClientAreaToServer();
   SendHitTestMaskToServer();
+
+  MusClient::Get()->OnCaptureClientSet(
+      aura::client::GetCaptureClient(window()));
 }
 
 std::unique_ptr<corewm::Tooltip> DesktopWindowTreeHostMus::CreateTooltip() {
@@ -332,6 +336,9 @@ void DesktopWindowTreeHostMus::Close() {
 }
 
 void DesktopWindowTreeHostMus::CloseNow() {
+  MusClient::Get()->OnCaptureClientUnset(
+      aura::client::GetCaptureClient(window()));
+
   native_widget_delegate_->OnNativeWidgetDestroying();
 
   // If we have children, close them. Use a copy for iteration because they'll