Move LayoutAndPaintAsyncThen to a separate compilation unit.

components/test_runner/layout_and_paint_async_then.h

Index: components/test_runner/layout_and_paint_async_then.h
diff --git a/components/test_runner/layout_and_paint_async_then.h b/components/test_runner/layout_and_paint_async_then.h
new file mode 100644
index 0000000000000000000000000000000000000000..be8ee491385d0609efba001eebb55a65ad94021d
--- /dev/null
+++ b/components/test_runner/layout_and_paint_async_then.h
@@ -0,0 +1,25 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_TEST_RUNNER_LAYOUT_AND_PAINT_ASYNC_THEN_H_
+#define COMPONENTS_TEST_RUNNER_LAYOUT_AND_PAINT_ASYNC_THEN_H_
+
+#include "base/callback_forward.h"
+#include "components/test_runner/test_runner_export.h"
+
+namespace blink {
+class WebWidget;
+}  // namespace blink
+
+namespace test_runner {
+
+// Triggers a layout and paint of |web_widget| and its popup (if any).
+// Calls |callback| after the layout and paint happens (for both the
+// |web_widget| and its popup (if any)).
+TEST_RUNNER_EXPORT void LayoutAndPaintAsyncThen(blink::WebWidget* web_widget,
+                                                const base::Closure& callback);
+
+}  // namespace test_runner
+
+#endif  // COMPONENTS_TEST_RUNNER_LAYOUT_AND_PAINT_ASYNC_THEN_H_