Add HeadlessFocusClient to fix document.hasFocus() issues.

headless/lib/browser/headless_web_contents_impl.cc

Index: headless/lib/browser/headless_web_contents_impl.cc
diff --git a/headless/lib/browser/headless_web_contents_impl.cc b/headless/lib/browser/headless_web_contents_impl.cc
index 9d4b7add3323b2db989e49f990c62d79346bf0ae..710fc84c4c9fe3849bf3da7f42ed82419ba44d11 100644
--- a/headless/lib/browser/headless_web_contents_impl.cc
+++ b/headless/lib/browser/headless_web_contents_impl.cc
@@ -21,6 +21,7 @@
 #include "content/public/browser/render_frame_host.h"
 #include "content/public/browser/render_process_host.h"
 #include "content/public/browser/render_view_host.h"
+#include "content/public/browser/render_widget_host.h"
 #include "content/public/browser/web_contents.h"
 #include "content/public/browser/web_contents_delegate.h"
 #include "content/public/common/bindings_policy.h"
@@ -97,6 +98,10 @@ class HeadlessWebContentsImpl::Delegate : public content::WebContentsDelegate {
                                             security_style_explanations);
   }
 
+  void ActivateContents(content::WebContents* contents) override {
+    contents->GetRenderViewHost()->GetWidget()->Focus();
+  }
+
  private:
   HeadlessBrowserContextImpl* browser_context_;  // Not owned.
   DISALLOW_COPY_AND_ASSIGN(Delegate);