Refactor MdFocusRing to make it easier to reuse on other controls.

ui/views/controls/combobox/combobox.cc

Index: ui/views/controls/combobox/combobox.cc
diff --git a/ui/views/controls/combobox/combobox.cc b/ui/views/controls/combobox/combobox.cc
index dfd1585cc2b7c2508c7565b7a3e08a22206d56fb..1d1fff2d0ac828b107ca399f9cef6020701bd86e 100644
--- a/ui/views/controls/combobox/combobox.cc
+++ b/ui/views/controls/combobox/combobox.cc
@@ -34,6 +34,7 @@
 #include "ui/views/controls/button/custom_button.h"
 #include "ui/views/controls/button/label_button.h"
 #include "ui/views/controls/combobox/combobox_listener.h"
+#include "ui/views/controls/focus_ring.h"
 #include "ui/views/controls/focusable_border.h"
 #include "ui/views/controls/menu/menu_config.h"
 #include "ui/views/controls/menu/menu_model_adapter.h"
@@ -392,12 +393,13 @@ class Combobox::ComboboxMenuModel : public ui::MenuModel,
 Combobox::Combobox(ui::ComboboxModel* model, Style style)
     : model_(model),
       style_(style),
-      listener_(NULL),
+      listener_(nullptr),
       selected_index_(style == STYLE_ACTION ? 0 : model_->GetDefaultIndex()),
       invalid_(false),
       menu_model_(new ComboboxMenuModel(this, model)),
       text_button_(new TransparentButton(this, style_ == STYLE_ACTION)),
       arrow_button_(new TransparentButton(this, style_ == STYLE_ACTION)),
+      focus_ring_(nullptr),
       size_to_largest_label_(style_ == STYLE_NORMAL),
       weak_ptr_factory_(this) {
   ModelChanged();
@@ -413,6 +415,8 @@ Combobox::Combobox(ui::ComboboxModel* model, Style style)
     std::unique_ptr<Background> b =
         PlatformStyle::CreateComboboxBackground(GetArrowContainerWidth());
     set_background(b.release());
+    focus_ring_ = new FocusRing();
+    AddChildView(focus_ring_);
   }
 
   // Initialize the button images.
@@ -715,6 +719,8 @@ void Combobox::OnFocus() {
   View::OnFocus();
   // Border renders differently when focused.
   SchedulePaint();
+  if (UseMd())
+    focus_ring_->SetVisible(true);
 }
 
 void Combobox::OnBlur() {
@@ -725,6 +731,8 @@ void Combobox::OnBlur() {
     selector_->OnViewBlur();
   // Border renders differently when focused.
   SchedulePaint();
+  if (UseMd())

[sky, 2016/10/03 23:53:15]

Is there a reason not to delegate focus_ring_ here?

[Evan Stade, 2016/10/04 01:04:55]

No reason.

Made a couple of statics to create and add/remove and destroy rings. (This also
addresses most of your other comments.)

+    focus_ring_->SetVisible(false);
 }
 
 void Combobox::GetAccessibleState(ui::AXViewState* state) {