Add support to previews/ for Server LoFi and LitePages

components/previews/core/previews_decider.h

 // 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_PREVIEWS_CORE_PREVIEWS_DECIDER_H_
 #define COMPONENTS_PREVIEWS_CORE_PREVIEWS_DECIDER_H_
 
 #include "components/previews/core/previews_experiments.h"
 
 namespace net {
 class URLRequest;
 }
 
 namespace previews {
 
 class PreviewsDecider {
  public:
-  // Whether |request| is allowed to show a preview of |type|.
+  // Whether |request| is allowed to show a preview of |type|. Previews that do
+  // not supply a network quality in
+  // GetEffectiveConnectionTypeThresholdForPreviewsType() should check the
+  // network quality before calling ShouldAllowPreview.
   virtual bool ShouldAllowPreview(const net::URLRequest& request,
                                   PreviewsType type) const = 0;
 
  protected:
   PreviewsDecider() {}
   virtual ~PreviewsDecider() {}
 };
 
 }  // namespace previews
 
 #endif  // COMPONENTS_PREVIEWS_CORE_PREVIEWS_DECIDER_H_