Try to get rid of all direct references to particular native themes.

ui/native_theme/native_theme_aura.cc

Index: ui/native_theme/native_theme_aura.cc
diff --git a/ui/native_theme/native_theme_aura.cc b/ui/native_theme/native_theme_aura.cc
index dc7166e6c491181340db32218f2f768dc48c7bab..20196a2cf3f9c6bf5695d41a316cc92a5044eafb 100644
--- a/ui/native_theme/native_theme_aura.cc
+++ b/ui/native_theme/native_theme_aura.cc
@@ -42,47 +42,43 @@ constexpr SkColor kOverlayScrollbarThumbColor[] = {SK_ColorBLACK,
 constexpr SkColor kOverlayScrollbarStrokeColor[] = {SK_ColorWHITE,
                                                     SK_ColorBLACK};
 
-SkAlpha ThumbAlphaForState(NativeTheme::State state) {
-  bool overlay = IsOverlayScrollbarEnabled();
-  switch (state) {
-    case NativeTheme::kDisabled:
-      return 0x00;
-    case NativeTheme::kHovered:
-      return overlay ? kOverlayScrollbarAlphaHovered : 0x4D;
-    case NativeTheme::kNormal:
-      return overlay ? kOverlayScrollbarAlphaNormal : 0x33;
-    case NativeTheme::kPressed:
-      return overlay ? kOverlayScrollbarAlphaPressed : 0x80;
-    case NativeTheme::kNumStates:
-      break;
-  }
-
-  NOTREACHED();
-  return 0xFF;
-}
-
 const SkColor kTrackColor = SkColorSetRGB(0xF1, 0xF1, 0xF1);
 
 }  // namespace
 
 // static
 NativeTheme* NativeTheme::GetInstanceForWeb() {
+  return NativeThemeAura::web_instance();
+}
+
+#if !defined(OS_WIN)
+// static
+NativeTheme* NativeTheme::GetInstanceForNativeUi() {
   return NativeThemeAura::instance();
 }
+#endif
 
 // static
 NativeThemeAura* NativeThemeAura::instance() {
-  CR_DEFINE_STATIC_LOCAL(NativeThemeAura, s_native_theme, ());
+  CR_DEFINE_STATIC_LOCAL(NativeThemeAura, s_native_theme, (false));
   return &s_native_theme;
 }
 
-NativeThemeAura::NativeThemeAura() {
+// static
+NativeThemeAura* NativeThemeAura::web_instance() {
+  CR_DEFINE_STATIC_LOCAL(NativeThemeAura, s_native_theme_for_web,
+                         (IsOverlayScrollbarEnabled()));
+  return &s_native_theme_for_web;
+}
+
+NativeThemeAura::NativeThemeAura(bool use_overlay_scrollbars)
+    : use_overlay_scrollbars_(use_overlay_scrollbars) {
 // We don't draw scrollbar buttons.
 #if defined(OS_CHROMEOS)
   set_scrollbar_button_length(0);
 #endif
 
-  if (IsOverlayScrollbarEnabled()) {
+  if (use_overlay_scrollbars_) {
     scrollbar_width_ =
         kOverlayScrollbarThumbWidthPressed + kOverlayScrollbarStrokeWidth * 2;
   }
@@ -174,7 +170,7 @@ void NativeThemeAura::PaintScrollbarTrack(
     const ScrollbarTrackExtraParams& extra_params,
     const gfx::Rect& rect) const {
   // Overlay Scrollbar should never paint a scrollbar track.
-  DCHECK(!IsOverlayScrollbarEnabled());
+  DCHECK(!use_overlay_scrollbars_);
   SkPaint paint;
   paint.setColor(kTrackColor);
   canvas->drawIRect(gfx::RectToSkIRect(rect), paint);
@@ -192,11 +188,24 @@ void NativeThemeAura::PaintScrollbarThumb(
 
   TRACE_EVENT0("blink", "NativeThemeAura::PaintScrollbarThumb");
 
+  SkAlpha thumb_alpha = SK_AlphaTRANSPARENT;
+  const bool overlay = use_overlay_scrollbars_;
+  switch (state) {
+    case NativeTheme::kDisabled:
+      thumb_alpha = SK_AlphaTRANSPARENT;
+    case NativeTheme::kHovered:
+      thumb_alpha = overlay ? kOverlayScrollbarAlphaHovered : 0x4D;
+    case NativeTheme::kNormal:
+      thumb_alpha = overlay ? kOverlayScrollbarAlphaNormal : 0x33;
+    case NativeTheme::kPressed:
+      thumb_alpha = overlay ? kOverlayScrollbarAlphaPressed : 0x80;
+    case NativeTheme::kNumStates:
+      break;
+  }
+
   gfx::Rect thumb_rect(rect);
   SkColor thumb_color;
-  SkAlpha thumb_alpha = ThumbAlphaForState(state);
-
-  if (IsOverlayScrollbarEnabled()) {
+  if (overlay) {
     thumb_color = kOverlayScrollbarThumbColor[theme];
 
     // In overlay mode, draw a stroke (border).
@@ -237,7 +246,7 @@ void NativeThemeAura::PaintScrollbarCorner(SkCanvas* canvas,
                                            State state,
                                            const gfx::Rect& rect) const {
   // Overlay Scrollbar should never paint a scrollbar corner.
-  DCHECK(!IsOverlayScrollbarEnabled());
+  DCHECK(!use_overlay_scrollbars_);
   SkPaint paint;
   paint.setColor(SkColorSetRGB(0xDC, 0xDC, 0xDC));
   canvas->drawIRect(RectToSkIRect(rect), paint);
@@ -246,7 +255,7 @@ void NativeThemeAura::PaintScrollbarCorner(SkCanvas* canvas,
 gfx::Size NativeThemeAura::GetPartSize(Part part,
                                        State state,
                                        const ExtraParams& extra) const {
-  if (IsOverlayScrollbarEnabled()) {
+  if (use_overlay_scrollbars_) {
     constexpr int minimum_length =
         kOverlayScrollbarMinimumLength + 2 * kOverlayScrollbarStrokeWidth;
 
@@ -257,12 +266,7 @@ gfx::Size NativeThemeAura::GetPartSize(Part part,
       case kScrollbarVerticalThumb:
         return gfx::Size(scrollbar_width_, minimum_length);
       default:
-        // TODO(bokan): We should probably make sure code using overlay
-        // scrollbars isn't asking for part sizes that don't exist. This
-        // currently breaks in Views layout code which indicates they aren't
-        // overlay aware yet. The Views code should be fixed and either this
-        // branch return 0 for parts that don't exist or assert NOTREACHED.
-        // crbug.com/657159.
+        NOTREACHED();
         break;
     }
   }