ui: Cleanup class/struct forward declarations

content/common/input/input_event_ack_source.h

Index: content/common/input/input_event_ack_source.h
diff --git a/content/common/input/input_event_ack_source.h b/content/common/input/input_event_ack_source.h
new file mode 100644
index 0000000000000000000000000000000000000000..c0cea996e0225dbd9d3a5acc352505c296711568
--- /dev/null
+++ b/content/common/input/input_event_ack_source.h
@@ -0,0 +1,21 @@
+// Copyright (c) 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 CONTENT_COMMON_INPUT_INPUT_EVENT_ACK_SOURCE_H_
+#define CONTENT_COMMON_INPUT_INPUT_EVENT_ACK_SOURCE_H_
+
+namespace content {
+
+// Describes the source of where the input event ACK was
+// generated from inside the renderer.
+enum class InputEventAckSource {
+  UNKNOWN,
+  COMPOSITOR_THREAD,
+  MAIN_THREAD,
+  MAX = MAIN_THREAD
+};
+
+}  // namespace content
+
+#endif  // CONTENT_COMMON_INPUT_INPUT_EVENT_ACK_SOURCE_H_