Clean up duplicate web app theme logic

This CL removes the system theme check for web apps in ShouldUseTheme().
This logic now lives in BrowserFrame::GetThemeProvider() as of:
https://chromium-review.googlesource.com/c/chromium/src/+/2239912

Bug: 1087693
Change-Id: Icd006bccab2fe08112990f8c856ecf4c75fef0c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2309993
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791041}

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

@@ -165,22 +165,6 @@ void BrowserFrame::OnBrowserViewInitViewsComplete() {
   browser_frame_view_->OnBrowserViewInitViewsComplete();
 }
 
-bool BrowserFrame::ShouldUseTheme() const {
-  // Browser windows are always themed (including popups).
-  if (!web_app::AppBrowserController::IsForWebAppBrowser(
-          browser_view_->browser())) {
-    return true;
-  }
-
-  // The system GTK theme should always be respected if the user has opted to
-  // use it.
-  if (IsUsingGtkTheme(browser_view_->browser()->profile()))
-    return true;
-
-  // Hosted apps on non-GTK use default colors.
-  return false;
-}
-
 ///////////////////////////////////////////////////////////////////////////////
 // BrowserFrame, views::Widget overrides:
 

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

@@ -111,9 +111,6 @@ class BrowserFrame : public views::Widget, public views::ContextMenuController {
   // Called when BrowserView creates all it's child views.
   void OnBrowserViewInitViewsComplete();
 
-  // Returns whether this window should be themed with the user's theme or not.
-  bool ShouldUseTheme() const;
-
   // views::Widget:
   views::internal::RootView* CreateRootView() override;
   std::unique_ptr<views::NonClientFrameView> CreateNonClientFrameView()

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

@@ -271,8 +271,8 @@ gfx::ImageSkia BrowserNonClientFrameView::GetFrameImage(
   const int frame_image_id = ShouldPaintAsActive(active_state)
                                  ? IDR_THEME_FRAME
                                  : IDR_THEME_FRAME_INACTIVE;
-  return frame_->ShouldUseTheme() && (tp->HasCustomImage(frame_image_id) ||
-                                      tp->HasCustomImage(IDR_THEME_FRAME))
+  return (tp->HasCustomImage(frame_image_id) ||
+          tp->HasCustomImage(IDR_THEME_FRAME))
              ? *tp->GetImageSkiaNamed(frame_image_id)
              : gfx::ImageSkia();
 }