Add HeadlessFocusClient to fix document.hasFocus() issues.

headless/lib/browser/headless_browser_impl_aura.cc

Index: headless/lib/browser/headless_browser_impl_aura.cc
diff --git a/headless/lib/browser/headless_browser_impl_aura.cc b/headless/lib/browser/headless_browser_impl_aura.cc
index 9237e347112052370d506943b67c40ee6c92cf23..4718d4914dcae1947b391ec541367877dd946698 100644
--- a/headless/lib/browser/headless_browser_impl_aura.cc
+++ b/headless/lib/browser/headless_browser_impl_aura.cc
@@ -6,7 +6,9 @@
 
 #include "content/public/browser/render_widget_host_view.h"
 #include "content/public/browser/web_contents.h"
+#include "headless/lib/browser/headless_focus_client.h"
 #include "headless/lib/browser/headless_screen.h"
+#include "ui/aura/client/focus_client.h"
 #include "ui/aura/env.h"
 #include "ui/aura/window.h"
 #include "ui/display/screen.h"
@@ -29,6 +31,10 @@ void HeadlessBrowserImpl::PlatformCreateWindow() {
   window_tree_host_->InitHost();
   window_tree_host_->window()->Show();
   window_tree_host_->SetParentWindow(window_tree_host_->window());
+
+  focus_client_.reset(new HeadlessFocusClient());
+  aura::client::SetFocusClient(window_tree_host_->window(),
+                               focus_client_.get());
 }
 
 void HeadlessBrowserImpl::PlatformInitializeWebContents(