Makes DefaultCaptureClient work with a null root_window

ui/aura/test/aura_test_helper.cc

Index: ui/aura/test/aura_test_helper.cc
diff --git a/ui/aura/test/aura_test_helper.cc b/ui/aura/test/aura_test_helper.cc
index 933cb2f240fb44619c036d6cdc49df5a0f838d06..76766aa7a82f81a10e0475c8fc153abd213212d7 100644
--- a/ui/aura/test/aura_test_helper.cc
+++ b/ui/aura/test/aura_test_helper.cc
@@ -78,7 +78,8 @@ void AuraTestHelper::SetUp(ui::ContextFactory* context_factory) {
   focus_client_.reset(new TestFocusClient);
   client::SetFocusClient(root_window(), focus_client_.get());
   parenting_client_.reset(new TestWindowParentingClient(root_window()));
-  capture_client_.reset(new client::DefaultCaptureClient(root_window()));
+  capture_client_ = base::MakeUnique<client::DefaultCaptureClient>();
+  SetCaptureClient(host_->window(), capture_client_.get());

[msw, 2016/10/25 19:21:08]

nit: client:: needed here like below?

[msw, 2016/10/25 19:21:08]

aside: odd that host_->window()'s root is not root_window()...
(I haven't looked closely to actually figure this out...)

[sky, 2016/10/25 19:44:40]

Surprisingly it isn't needed here, but is needed below. That is, this compiles,
but if I remove client:: from below I get:

../../ui/aura/test/aura_test_helper.cc:92:3: error: use of undeclared identifier
'SetCaptureClient'; did you mean 'client::SetCaptureClient'?
  SetCaptureClient(host_->window(), nullptr);
  ^~~~~~~~~~~~~~~~
  client::SetCaptureClient
../../ui/aura/client/capture_client.h:44:18: note: 'client

I stepped through this code to ensure it calls to aura::client::SetCaptureClient
and it does. I'm speculating, but I'm guessing having a type in capture_client_
makes this call work without the client:: where as the code below has no type,
so it fails.

[msw, 2016/10/25 19:47:59]

Odd... I might add it to be clear/consistent, but it's up to you.

 
   root_window()->Show();
   // Ensure width != height so tests won't confuse them.
@@ -88,6 +89,7 @@ void AuraTestHelper::SetUp(ui::ContextFactory* context_factory) {
 void AuraTestHelper::TearDown() {
   teardown_called_ = true;
   parenting_client_.reset();
+  client::SetCaptureClient(host_->window(), nullptr);
   capture_client_.reset();
   focus_client_.reset();
   client::SetFocusClient(root_window(), nullptr);