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..0178717edeade54d8de21bdf0913ee4ce7740f17 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 +41,14 @@ 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_) {
+    capture_delegate = capture_window_->GetHost()->dispatcher();

[msw, 2016/10/25 17:57:58]

q: just checking that the change from |root_window_| to |capture_window_| and
|old_capture_window_| to get the WindowTreeHost and WindowEventDispatcher is
intentional; should we check that the root of the [old] capture window matches
|root_window_| if that value is non-null?

     capture_delegate->SetNativeCapture();
-  else
+  } else {
+    capture_delegate = old_capture_window->GetHost()->dispatcher();
     capture_delegate->ReleaseNativeCapture();
+  }
 
   capture_delegate->UpdateCapture(old_capture_window, capture_window_);