Change CaptureSynchronizer and PointerWatcherEventRouter to support multiple CaptureClients.

ui/aura/client/capture_client.h

Index: ui/aura/client/capture_client.h
diff --git a/ui/aura/client/capture_client.h b/ui/aura/client/capture_client.h
index c7874267e8b0645d48631d97b7067f6407616109..bbabbe2a2b075c79f21ad81e9ab4b44c2c222c6b 100644
--- a/ui/aura/client/capture_client.h
+++ b/ui/aura/client/capture_client.h
@@ -6,6 +6,7 @@
 #define UI_AURA_CLIENT_CAPTURE_CLIENT_H_
 
 #include "ui/aura/aura_export.h"
+#include "ui/aura/window.h"
 
 namespace aura {
 class Window;
@@ -49,6 +50,10 @@ AURA_EXPORT CaptureClient* GetCaptureClient(Window* root_window);
 // if the window doesn't have a root window, or there is no capture window.
 AURA_EXPORT Window* GetCaptureWindow(Window* window);
 
+// The capture client is stored on the window using this property.
+AURA_EXPORT extern const WindowProperty<CaptureClient*>* const
+    kRootWindowCaptureClientKey;
+
 }  // namespace client
 }  // namespace aura