MacViews: Handle Space and Return keys correctly for Buttons.

ui/views/style/platform_style.h

Index: ui/views/style/platform_style.h
diff --git a/ui/views/style/platform_style.h b/ui/views/style/platform_style.h
index e71720fbce3b714c30ebabab18757260a0eec5ac..6b5817cf86dbfd346a9bcb705ed3c9dd9b086582 100644
--- a/ui/views/style/platform_style.h
+++ b/ui/views/style/platform_style.h
@@ -46,6 +46,12 @@ class VIEWS_EXPORT PlatformStyle {
   // The menu button's action to show the menu.
   static const CustomButton::NotifyAction kMenuNotifyActivationAction;
 
+  // Click action to perform on a button when the Space key is pressed.

[tapted, 2017/01/05 00:34:41]

This comment doesn't read quite right, since "pressed" implies only one side of
the action.

Maybe something like

// Whether the space key clicks a button on key press or key release.

[karandeepb, 2017/01/05 10:50:09]

Done.

+  static const CustomButton::ClickAction kClickActionOnSpace;
+
+  // Whether Return key clicks the focused control.

[tapted, 2017/01/05 00:34:41]

It might be good to be explicit here too. E.g. 

"Whether the Return key clicks the focused control (on key press). Otherwise,
Return does nothing unless it is handled by an accelerator."

(actually on Mac, it should NSBeep() if not handled by an accelerator, but
that's orthogonal)

[karandeepb, 2017/01/05 10:50:09]

Done.

+  static const bool kReturnClicksFocusedControl;
+
   // Whether TreeViews get a focus ring on the entire TreeView when focused.
   static const bool kTreeViewHasFocusRing;