Makes DefaultCaptureClient work with a null root_window

ui/aura/client/default_capture_client.cc

Index: ui/aura/client/default_capture_client.cc
diff --git a/ui/aura/client/default_capture_client.cc b/ui/aura/client/default_capture_client.cc
index 19f0d40c2a0127b1e21fa39d88db835983db2abd..5132431d380f4888ec369f79b39b8c064dad2438 100644
--- a/ui/aura/client/default_capture_client.cc
+++ b/ui/aura/client/default_capture_client.cc
@@ -20,13 +20,15 @@ Window* global_capture_window_ = nullptr;
 
 DefaultCaptureClient::DefaultCaptureClient(Window* root_window)
     : root_window_(root_window), capture_window_(nullptr) {
-  SetCaptureClient(root_window_, this);
+  if (root_window_)
+    SetCaptureClient(root_window_, this);
 }
 
 DefaultCaptureClient::~DefaultCaptureClient() {
   if (global_capture_window_ == capture_window_)
     global_capture_window_ = nullptr;
-  SetCaptureClient(root_window_, nullptr);
+  if (root_window_)
+    SetCaptureClient(root_window_, nullptr);
 }
 
 void DefaultCaptureClient::SetCapture(Window* window) {
@@ -39,11 +41,17 @@ void DefaultCaptureClient::SetCapture(Window* window) {
   capture_window_ = window;
   global_capture_window_ = window;
 
-  CaptureDelegate* capture_delegate = root_window_->GetHost()->dispatcher();
-  if (capture_window_)
+  CaptureDelegate* capture_delegate = nullptr;
+  if (capture_window_) {
+    DCHECK(!root_window_ || root_window_ == capture_window_->GetRootWindow());
+    capture_delegate = capture_window_->GetHost()->dispatcher();
     capture_delegate->SetNativeCapture();
-  else
+  } else {
+    DCHECK(!root_window_ ||
+           root_window_ == old_capture_window->GetRootWindow());
+    capture_delegate = old_capture_window->GetHost()->dispatcher();
     capture_delegate->ReleaseNativeCapture();
+  }
 
   capture_delegate->UpdateCapture(old_capture_window, capture_window_);