Add support to previews/ for Server LoFi and LitePages

components/previews/core/previews_decider.h

Index: components/previews/core/previews_decider.h
diff --git a/components/previews/core/previews_decider.h b/components/previews/core/previews_decider.h
index f293c87c1ad0f621e85cf0756f6872cf0c2f6c60..643a4aeffbba954dd7ccde00c4df5ffe5562ddad 100644
--- a/components/previews/core/previews_decider.h
+++ b/components/previews/core/previews_decider.h
@@ -15,7 +15,10 @@ 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;