Assume all providers may give zero suggest responses.

components/omnibox/autocomplete_provider.h

 // Copyright 2014 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_OMNIBOX_AUTOCOMPLETE_PROVIDER_H_
 #define COMPONENTS_OMNIBOX_AUTOCOMPLETE_PROVIDER_H_
 
 #include "base/basictypes.h"
 #include "base/memory/ref_counted.h"
 #include "base/strings/string16.h"
@@ skipping 138 matching lines @@
   // is, synchronously) do not cause any notifications to be sent.  The
   // controller is expected to check for these without prompting (since
   // otherwise, starting each provider running would result in a flurry of
   // notifications).
   //
   // Once Stop() has been called, no more notifications should be sent.
   //
   // |minimal_changes| is an optimization that lets the provider do less work
   // when the |input|'s text hasn't changed.  See the body of
   // OmniboxPopupModel::StartAutocomplete().
-  virtual void Start(const AutocompleteInput& input, bool minimal_changes) = 0;
+  //
+  // |on_focus| is set to true when autocomplete provider is triggered through
+  // omnibox focus rather than input and should only return matches expected
+  // on focus.

[Peter Kasting, 2015/01/07 20:16:36]

How about this:

  // |called_due_to_focus| is true when Start() is being called in response to
  // the omnibox being focused, instead of due to e.g. user input changes.  Most
  // providers should not provide matches in this case.  Providers which want to
  // display matches on focus can use this flag to know when they can do so.
  virtual void Start(const AutocompleteInput& input,
                     bool minimal_changes,
                     bool called_due_to_focus);

[Maria, 2015/01/08 07:52:38]

Done.

+  virtual void Start(const AutocompleteInput& input,
+                     bool minimal_changes,
+                     bool on_focus) = 0;
 
   // Called when a provider must not make any more callbacks for the current
   // query. This will be called regardless of whether the provider is already
   // done.  If the provider caches any results, it should clear the cache based
   // on the value of |clear_cached_results|.
   virtual void Stop(bool clear_cached_results);
 
   // Returns the enum equivalent to the name of this provider.
   // TODO(derat): Make metrics use AutocompleteProvider::Type directly, or at
   // least move this method to the metrics directory.
@@ skipping 20 matching lines @@
 
   // Returns the set of matches for the current query.
   const ACMatches& matches() const { return matches_; }
 
   // Returns whether the provider is done processing the query.
   bool done() const { return done_; }
 
   // Returns this provider's type.
   Type type() const { return type_; }
 
-  // Returns whether the provider wants to provide matches in response to the
-  // omnibox being focused.
-  virtual bool ProvidesMatchesOnOmniboxFocus() const;
-
   // Returns a string describing this provider's type.
   const char* GetName() const;
 
   // A suggested upper bound for how many matches a provider should return.
   // TODO(pkasting): http://b/1111299 , http://b/933133 This should go away once
   // we have good relevance heuristics; the controller should handle all
   // culling.
   static const size_t kMaxMatches;
 
  protected:
@@ skipping 29 matching lines @@
   ACMatches matches_;
   bool done_;
 
   Type type_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(AutocompleteProvider);
 };
 
 #endif  // COMPONENTS_OMNIBOX_AUTOCOMPLETE_PROVIDER_H_