Assume all providers may give zero suggest responses.

components/omnibox/autocomplete_provider.h

Index: components/omnibox/autocomplete_provider.h
diff --git a/components/omnibox/autocomplete_provider.h b/components/omnibox/autocomplete_provider.h
index 96102928a62bb1f3153d3a541f0d7916e0296a2d..f209e43a2f8077a2681763778e78ef75597fb132 100644
--- a/components/omnibox/autocomplete_provider.h
+++ b/components/omnibox/autocomplete_provider.h
@@ -156,7 +156,13 @@ class AutocompleteProvider
   // |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
@@ -197,10 +203,6 @@ class AutocompleteProvider
   // 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;