MacViews: Fix mac_views_browser compilation due to unused variable on Mac.

chrome/browser/ui/views/profiles/avatar_button.cc

Index: chrome/browser/ui/views/profiles/avatar_button.cc
diff --git a/chrome/browser/ui/views/profiles/avatar_button.cc b/chrome/browser/ui/views/profiles/avatar_button.cc
index 20d0f3f343711f5a79205326c3eab0cdbc2025e8..60f417cea85ca7ef4ba3093c4938e755f67b6a0c 100644
--- a/chrome/browser/ui/views/profiles/avatar_button.cc
+++ b/chrome/browser/ui/views/profiles/avatar_button.cc
@@ -200,13 +200,14 @@ AvatarButton::AvatarButton(views::ButtonListener* listener,
   if (apply_ink_drop) {
     SetInkDropMode(InkDropMode::ON);
     SetFocusPainter(nullptr);
-    constexpr int kIconSize = 16;
 #if defined(OS_LINUX)
+    constexpr int kIconSize = 16;
     set_ink_drop_base_color(SK_ColorWHITE);
     SetBorder(base::MakeUnique<AvatarButtonThemedBorder>());
     generic_avatar_ = gfx::CreateVectorIcon(kProfileSwitcherOutlineIcon,
                                             kIconSize, gfx::kPlaceholderColor);
 #elif defined(OS_WIN)
+    constexpr int kIconSize = 16;
     DCHECK_EQ(AvatarButtonStyle::NATIVE, button_style);
     set_ink_drop_base_color(SK_ColorBLACK);
     SetBorder(views::CreateEmptyBorder(kBorderInsets));