[headless] Use individual aura::WindowTreeHosts per WebContents.

headless/lib/browser/headless_window_tree_host.cc

Index: headless/lib/browser/headless_window_tree_host.cc
diff --git a/headless/lib/browser/headless_window_tree_host.cc b/headless/lib/browser/headless_window_tree_host.cc
index 5cae74e7d48a2cf1e5d37efe3b82211db4444035..6b59e30d47d93053d3da4b7572c0c6b7f5151a83 100644
--- a/headless/lib/browser/headless_window_tree_host.cc
+++ b/headless/lib/browser/headless_window_tree_host.cc
@@ -3,8 +3,10 @@
 // found in the LICENSE file.
 
 #include "headless/lib/browser/headless_window_tree_host.h"
-#include "ui/aura/window.h"
 
+#include "headless/lib/browser/headless_focus_client.h"
+#include "headless/lib/browser/headless_window_parenting_client.h"
+#include "ui/aura/window.h"
 #include "ui/gfx/icc_profile.h"
 
 namespace headless {
@@ -13,6 +15,9 @@ HeadlessWindowTreeHost::HeadlessWindowTreeHost(const gfx::Rect& bounds)
     : bounds_(bounds) {
   CreateCompositor();
   OnAcceleratedWidgetAvailable();
+
+  focus_client_.reset(new HeadlessFocusClient());
+  aura::client::SetFocusClient(window(), focus_client_.get());
 }
 
 HeadlessWindowTreeHost::~HeadlessWindowTreeHost() {