[MacViews] Show combobox menu popup at mouse press

ui/views/style/platform_style_mac.mm

Index: ui/views/style/platform_style_mac.mm
diff --git a/ui/views/style/platform_style_mac.mm b/ui/views/style/platform_style_mac.mm
index 5a8d2bd0b6322c0489fc5eb8c1e7ebb54362d9f0..96b5d405c64c8be2f64616b49e16d57e02847b78 100644
--- a/ui/views/style/platform_style_mac.mm
+++ b/ui/views/style/platform_style_mac.mm
@@ -24,6 +24,9 @@ const int PlatformStyle::kMinLabelButtonHeight = 30;
 const bool PlatformStyle::kDefaultLabelButtonHasBoldFont = false;
 const bool PlatformStyle::kTextfieldDragVerticallyDragsToEnd = true;
 
+const CustomButton::NotifyAction PlatformStyle::kMenuActivationAction =
+    CustomButton::NOTIFY_ON_PRESS;
+
 // static
 gfx::ImageSkia PlatformStyle::CreateComboboxArrow(bool is_enabled,
                                                   Combobox::Style style) {