Use correct ScrollbarTheme to paintScrollCorner

third_party/WebKit/Source/core/paint/FramePainter.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/FramePainter.h"
 
 #include "core/editing/markers/DocumentMarkerController.h"
 #include "core/frame/FrameView.h"
 #include "core/inspector/InspectorInstrumentation.h"
 #include "core/inspector/InspectorTraceEvents.h"
@@ skipping 210 matching lines @@
           context, *frameView().layoutView(), DisplayItem::kScrollbarCorner,
           FloatRect(cornerRect));
       context.fillRect(cornerRect, frameView().baseBackgroundColor());
     }
     ScrollbarPainter::paintIntoRect(*frameView().scrollCorner(), context,
                                     cornerRect.location(),
                                     LayoutRect(cornerRect));
     return;
   }
 
-  ScrollbarTheme::theme().paintScrollCorner(context, *frameView().layoutView(),
-                                            cornerRect);
+  ScrollbarTheme* theme = nullptr;
+
+  if (frameView().horizontalScrollbar()) {
+    theme = &frameView().horizontalScrollbar()->theme();
+  } else if (frameView().verticalScrollbar()) {
+    theme = &frameView().verticalScrollbar()->theme();
+  } else {
+    NOTREACHED();
+    theme = &ScrollbarTheme::theme();

[bokan, 2017/03/13 21:33:52]

style guide says don't handle failing DCHECKs:
https://chromium.googlesource.com/chromium/src/+/master/styleguide/c++/c++.md...

So if you're confident we never reach this case (I think we should only ever
paint a scroll corner if we have both scrollbars) then remove the theme = ...
line. If there are cases where we hit this then remove the NOTREACHED.

+  }
+
+  theme->paintScrollCorner(context, *frameView().layoutView(), cornerRect);
 }
 
 void FramePainter::paintScrollbar(GraphicsContext& context,
                                   Scrollbar& bar,
                                   const IntRect& rect) {
   bool needsBackground =
       bar.isCustomScrollbar() && frameView().frame().isMainFrame();
   if (needsBackground) {
     IntRect toFill = bar.frameRect();
     toFill.intersect(rect);
     context.fillRect(toFill, frameView().baseBackgroundColor());
   }
 
   bar.paint(context, CullRect(rect));
 }
 
 const FrameView& FramePainter::frameView() {
   DCHECK(m_frameView);
   return *m_frameView;
 }
 
 }  // namespace blink