Fix GTK native theming for non-browser windows

This fixes a regression after [1].  We want to continue to use the
LinuxUI instance for getting the NativeTheme for all windows, not just
browser windows.

[1] https://crrev.com/c/2803073

R=sky

Bug: 1203346
Change-Id: I26cb9610e1f14927b60083ccd27ce408cafc7be9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2872864
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#879192}

chrome/browser/ui/views/frame/browser_frame.cc

@@ -46,7 +46,6 @@
 // of lacros-chrome is complete.
 #if defined(OS_LINUX) || BUILDFLAG(IS_CHROMEOS_LACROS)
 #include "ui/display/screen.h"
-#include "ui/views/linux_ui/linux_ui.h"
 #endif
 
 namespace {
@@ -355,12 +354,6 @@ void BrowserFrame::SelectNativeTheme() {
     }
   }
 
-#if defined(OS_LINUX) || defined(IS_CHROMEOS_LACROS)
-  const views::LinuxUI* linux_ui = views::LinuxUI::instance();
-  if (linux_ui)
-    native_theme = linux_ui->GetNativeTheme(GetNativeWindow());
-#endif
-
   SetNativeTheme(native_theme);
 }
 

ui/views/widget/widget.cc

@@ -47,6 +47,10 @@
 #include "ui/views/window/custom_frame_view.h"
 #include "ui/views/window/dialog_delegate.h"
 
+#if defined(OS_LINUX)
+#include "ui/views/linux_ui/linux_ui.h"
+#endif
+
 namespace views {
 
 namespace {
@@ -1647,6 +1651,13 @@ const ui::NativeTheme* Widget::GetNativeTheme() const {
       parent_)
     return parent_->GetNativeTheme();
 
+#if defined(OS_LINUX)
+  if (const views::LinuxUI* linux_ui = views::LinuxUI::instance()) {
+    if (auto* native_theme = linux_ui->GetNativeTheme(GetNativeWindow()))
+      return native_theme;
+  }
+#endif
+
   return ui::NativeTheme::GetInstanceForNativeUi();
 }