Revert of Revert of Move preferred scrollbar style preference change listening from renderer to browser, 3 of 3.

Source/platform/scroll/ScrollbarThemeMacCommon.mm

Index: Source/platform/scroll/ScrollbarThemeMacCommon.mm
diff --git a/Source/platform/scroll/ScrollbarThemeMacCommon.mm b/Source/platform/scroll/ScrollbarThemeMacCommon.mm
index f111c063ad33d45ecdc08e1b22bc0983c4dfb929..402d99aa23091b5f68ae005b92ace64a5717dd2d 100644
--- a/Source/platform/scroll/ScrollbarThemeMacCommon.mm
+++ b/Source/platform/scroll/ScrollbarThemeMacCommon.mm
@@ -24,6 +24,7 @@
  */
 
 #include "config.h"
+#include "RuntimeEnabledFeatures.h"
 #include "platform/scroll/ScrollbarThemeMacCommon.h"
 
 #include <Carbon/Carbon.h>
@@ -74,13 +75,14 @@
 static float gInitialButtonDelay = 0.5f;
 static float gAutoscrollButtonDelay = 0.05f;
 static bool gJumpOnTrackClick = false;
+static NSScrollerStyle gPreferredScrollerStyle = NSScrollerStyleLegacy;
 
 ScrollbarTheme* ScrollbarTheme::nativeTheme()
 {
     static ScrollbarThemeMacCommon* theme = NULL;
     if (theme)
         return theme;
-    if (isScrollbarOverlayAPIAvailable()) {
+    if (ScrollbarThemeMacCommon::isOverlayAPIAvailable()) {
         DEFINE_STATIC_LOCAL(ScrollbarThemeMacOverlayAPI, overlayTheme, ());
         theme = &overlayTheme;
     } else {
@@ -291,18 +293,26 @@
 {
 }
 
-void ScrollbarThemeMacCommon::preferencesChanged(float initialButtonDelay, float autoscrollButtonDelay, bool jumpOnTrackClick, bool redraw)
+void ScrollbarThemeMacCommon::preferencesChanged(float initialButtonDelay, float autoscrollButtonDelay, bool jumpOnTrackClick, NSScrollerStyle preferredScrollerStyle, bool redraw)
 {
     updateButtonPlacement();
     gInitialButtonDelay = initialButtonDelay;
     gAutoscrollButtonDelay = autoscrollButtonDelay;
     gJumpOnTrackClick = jumpOnTrackClick;
+    bool sendScrollerStyleNotification = gPreferredScrollerStyle != preferredScrollerStyle;
+    gPreferredScrollerStyle = preferredScrollerStyle;
     if (redraw && !scrollbarSet().isEmpty()) {
         ScrollbarSet::iterator end = scrollbarSet().end();
         for (ScrollbarSet::iterator it = scrollbarSet().begin(); it != end; ++it) {
             (*it)->styleChanged();
             (*it)->invalidate();
         }
+    }
+    if (sendScrollerStyleNotification) {
+        [[NSNotificationCenter defaultCenter]
+            postNotificationName:@"NSPreferredScrollerStyleDidChangeNotification"
+                          object:nil
+                        userInfo:@{ @"NSScrollerStyle" : @(gPreferredScrollerStyle) }];
     }
 }
 
@@ -348,4 +358,21 @@
     }
 }
 
+// static
+NSScrollerStyle ScrollbarThemeMacCommon::recommendedScrollerStyle()
+{
+    if (RuntimeEnabledFeatures::overlayScrollbarsEnabled())
+        return NSScrollerStyleOverlay;
+    return gPreferredScrollerStyle;
+}
+
+// static
+bool ScrollbarThemeMacCommon::isOverlayAPIAvailable()
+{
+    static bool apiAvailable =
+        [NSClassFromString(@"NSScrollerImp") respondsToSelector:@selector(scrollerImpWithStyle:controlSize:horizontal:replacingScrollerImp:)]
+        && [NSClassFromString(@"NSScrollerImpPair") instancesRespondToSelector:@selector(scrollerStyle)];
+    return apiAvailable;
+}
+
 } // namespace WebCore