Adds Lo-Fi fallback support for new Data Reduction Proxy protocol.

content/renderer/previews_state_helper.h

Index: content/renderer/previews_state_helper.h
diff --git a/content/renderer/previews_state_helper.h b/content/renderer/previews_state_helper.h
new file mode 100644
index 0000000000000000000000000000000000000000..c4050953e74aec4a8ce7f784107f47c0095b3979
--- /dev/null
+++ b/content/renderer/previews_state_helper.h
@@ -0,0 +1,21 @@
+// Copyright 2017 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_RENDERER_PREVIEWS_STATE_HELPER_H_
+#define CONTENT_RENDERER_PREVIEWS_STATE_HELPER_H_
+
+#include "content/public/common/previews_state.h"
+#include "third_party/WebKit/public/platform/WebURLResponse.h"
+
+namespace content {
+
+// Determines and returns an updated PreviewsState value for a main frame
+// based on its |original_state| value and its |web_url_response|.
+CONTENT_EXPORT PreviewsState GetPreviewsStateFromMainFrameResponse(
+    PreviewsState original_state,
+    const blink::WebURLResponse& web_url_response);
+
+}  // namespace content
+
+#endif  // CONTENT_RENDERER_PREVIEWS_STATE_HELPER_H_