Remove unused references to NativeTheme in TrayPopupItemStyle.

ash/common/system/chromeos/network/network_list_md.cc

Index: ash/common/system/chromeos/network/network_list_md.cc
diff --git a/ash/common/system/chromeos/network/network_list_md.cc b/ash/common/system/chromeos/network/network_list_md.cc
index 12b2f19708e95453bd1b3c02e5a57551faf07aac..b91d6eacff82af46f21fafee6422c4b42749d584 100644
--- a/ash/common/system/chromeos/network/network_list_md.cc
+++ b/ash/common/system/chromeos/network/network_list_md.cc
@@ -208,7 +208,8 @@ class WifiHeaderRowView : public NetworkListViewMd::SectionHeaderRowView {
   }
 
   void AddExtraButtons(bool enabled) override {
-    const SkColor prominent_color = style()->GetIconColor();
+    const SkColor prominent_color = GetNativeTheme()->GetSystemColor(
+        ui::NativeTheme::kColorId_ProminentButtonColor);
     gfx::ImageSkia normal_image = network_icon::GetImageForNewWifiNetwork(
         SkColorSetA(prominent_color, kJoinIconAlpha),
         SkColorSetA(prominent_color, kJoinBadgeAlpha));