Revert: Remove WindowTreeClientDelegate::GetCaptureClient() and WindowTreeClient::GetCaptureClient()

mash/simple_wm/simple_wm.cc

Index: mash/simple_wm/simple_wm.cc
diff --git a/mash/simple_wm/simple_wm.cc b/mash/simple_wm/simple_wm.cc
index b8f98a75bcdb6be7fd1b2567edb6297f55605601..90fd986ee6b97d002636a0ebacd6fba590b6c32c 100644
--- a/mash/simple_wm/simple_wm.cc
+++ b/mash/simple_wm/simple_wm.cc
@@ -399,6 +399,10 @@
   // Don't care.
 }
 
+aura::client::CaptureClient* SimpleWM::GetCaptureClient() {
+  return wm_state_.capture_controller();
+}
+
 aura::PropertyConverter* SimpleWM::GetPropertyConverter() {
   return &property_converter_;
 }