MacViews: Implement Full Keyboard Access.

ui/views/controls/textfield/textfield.cc

Index: ui/views/controls/textfield/textfield.cc
diff --git a/ui/views/controls/textfield/textfield.cc b/ui/views/controls/textfield/textfield.cc
index 44e1cf9b200cc3c09bddf918a134b68ba8961cf7..1141cac426fd1c4eb3db7043ac65de333bba7f39 100644
--- a/ui/views/controls/textfield/textfield.cc
+++ b/ui/views/controls/textfield/textfield.cc
@@ -294,6 +294,9 @@ Textfield::Textfield()
   set_drag_controller(this);
   GetRenderText()->SetFontList(GetDefaultFontList());
   SetBorder(std::unique_ptr<Border>(new FocusableBorder()));
+
+  // On Mac, only textfields and list controls are focusable by default. Hence

[tapted, 2016/04/20 06:05:58]

Same here. E.g. saying "only <foo>" suggests we're about to make it _not_
focusable, because we're not a <foo>. But this is a <foo>.

This probably doesn't need a comment at all. A textfield without focus is a
weird concept on any platform.

[karandeepb, 2016/05/03 02:54:13]

Done.

+  // use ALWAYS focus behavior.
   SetFocusBehavior(FocusBehavior::ALWAYS);
 
   if (ViewsDelegate::GetInstance()) {