MenuButton: support Mac look & feel

ui/views/style/platform_style.cc

Index: ui/views/style/platform_style.cc
diff --git a/ui/views/style/platform_style.cc b/ui/views/style/platform_style.cc
index 9027c6632dd0f0365d3ca45922c63e357c996c3c..4466fff3afc42a055d3c76f021b5b0297003c6cd 100644
--- a/ui/views/style/platform_style.cc
+++ b/ui/views/style/platform_style.cc
@@ -56,6 +56,16 @@ std::unique_ptr<Background> PlatformStyle::CreateComboboxBackground() {
 }
 
 // static
+std::unique_ptr<Background> PlatformStyle::CreateMenuButtonBackground() {
+  return nullptr;
+}
+
+// static
+std::unique_ptr<FocusableBorder> PlatformStyle::CreateMenuButtonBorder() {
+  return base::WrapUnique(new FocusableBorder());
+}
+
+// static
 std::unique_ptr<LabelButtonBorder> PlatformStyle::CreateLabelButtonBorder(
     Button::ButtonStyle style) {
   if (!ui::MaterialDesignController::IsModeMaterial() ||
@@ -70,6 +80,12 @@ std::unique_ptr<LabelButtonBorder> PlatformStyle::CreateLabelButtonBorder(
 }
 
 // static
+gfx::ImageSkia PlatformStyle::CreateMenuButtonArrow(bool is_enabled) {
+  ui::ResourceBundle& rb = ui::ResourceBundle::GetSharedInstance();
+  return *rb.GetImageSkiaNamed(IDR_MENU_DROPARROW);
+}
+
+// static
 std::unique_ptr<ScrollBar> PlatformStyle::CreateScrollBar(bool is_horizontal) {
   return base::WrapUnique(new NativeScrollBar(is_horizontal));
 }
@@ -83,6 +99,24 @@ SkColor PlatformStyle::TextColorForButton(
 
 #endif  // OS_MACOSX
 
+// This definition is used even on Mac, but Mac has its own
+// CreateComboboxArrow() so it will return something different on Mac.
+// static
+int PlatformStyle::GetComboboxShoulderWidth(Combobox::Style style) {
+  const int kNormalPadding = 7;
+  const int kActionPadding = 11;
+  int padding =
+      style == Combobox::STYLE_NORMAL ? kNormalPadding * 2 : kActionPadding * 2;
+  int image_width = CreateComboboxArrow(true, style).size().width();

[tapted, 2016/04/22 13:46:52]

I don't think the .size() is needed, same below. But, I also think we should
hard-code this, and perhaps DCHECK against the width(), since this gets called
quite a bit, and a resource bundle lookup has some cost (as does copying the
ImageSkia - it's ref-counted, but thread safe so there's special instructions
and impact to the cpu cache and stuff).

but of course that would need different widths based on the platform. (maybe
it's better to keep the ImageSkia on the Combobox instance and initialize it in
the constructor)

[Elly Fong-Jones, 2016/04/25 14:30:51]

Okay.

The reason why it wasn't statically computed right now is because SetStyle() can
get called on the combobox later on. I'm refactoring Combobox so that the style
is passed into the constructor and is const, and then I can cache the arrow
image and width inside the combobox. I made the style const and cached the image
inside Combobox/MenuButton, then kicked the Get*ShoulderWidth() methods back
into the classes themselves, so the only thing PlatformStyle exports is the
Get*ArrowImage() methods, which are called only once in the constructors. This
also makes the code easier to follow, so win/win.

+  return padding + image_width;
+}
+
+// static
+int PlatformStyle::GetMenuButtonShoulderWidth() {
+  const int kPadding = 7;
+  return kPadding * 2 + CreateMenuButtonArrow(true).size().width();
+}
+
 #if !defined(DESKTOP_LINUX) && !defined(OS_MACOSX)
 // static
 void PlatformStyle::ApplyLabelButtonTextStyle(