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

Source/platform/mac/NSScrollerImpDetails.mm

Index: Source/platform/mac/NSScrollerImpDetails.mm
diff --git a/Source/platform/mac/NSScrollerImpDetails.mm b/Source/platform/mac/NSScrollerImpDetails.mm
new file mode 100644
index 0000000000000000000000000000000000000000..703ce2f42e9d37c87a9bb519175c7b4242b5c74d
--- /dev/null
+++ b/Source/platform/mac/NSScrollerImpDetails.mm
@@ -0,0 +1,95 @@
+/*
+ * Copyright (C) 2011 Apple Inc. All Rights Reserved.
+ * Copyright (C) 2011 Google Inc. All Rights Reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+
+#include "config.h"
+#include "RuntimeEnabledFeatures.h"
+#include "platform/mac/NSScrollerImpDetails.h"
+
+namespace {
+
+// Declare notification names from the 10.7 SDK.
+#if !defined(MAC_OS_X_VERSION_10_7) || MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_7
+NSString* NSPreferredScrollerStyleDidChangeNotification = @"NSPreferredScrollerStyleDidChangeNotification";
+#endif
+
+// Storing the current NSScrollerStyle as a global is appreciably faster than
+// having it be a property of ScrollerStylerObserver.
+NSScrollerStyle g_scrollerStyle = NSScrollerStyleLegacy;
+
+}  // anonymous namespace
+
+@interface ScrollerStyleObserver : NSObject
+- (id)init;
+- (void)preferredScrollerStyleDidChange:(NSNotification*)notification;
+@end
+
+@implementation ScrollerStyleObserver
+- (id)init
+{
+    if ((self = [super init])) {
+        if ([NSScroller respondsToSelector:@selector(preferredScrollerStyle)]) {
+            g_scrollerStyle = [NSScroller preferredScrollerStyle];
+            NSNotificationCenter* center = [NSNotificationCenter defaultCenter];
+            [center addObserver:self
+                       selector:@selector(preferredScrollerStyleDidChange:)
+                           name:NSPreferredScrollerStyleDidChangeNotification
+                         object:nil];
+        }
+    }
+    return self;
+}
+
+- (void)preferredScrollerStyleDidChange:(NSNotification*)notification
+{
+    g_scrollerStyle = [NSScroller preferredScrollerStyle];
+}
+@end
+
+namespace WebCore {
+
+bool isScrollbarOverlayAPIAvailable()
+{
+    static bool apiAvailable =
+        [NSClassFromString(@"NSScrollerImp") respondsToSelector:@selector(scrollerImpWithStyle:controlSize:horizontal:replacingScrollerImp:)]
+        && [NSClassFromString(@"NSScrollerImpPair") instancesRespondToSelector:@selector(scrollerStyle)];
+    return apiAvailable;
+}
+
+NSScrollerStyle recommendedScrollerStyle()
+{
+    if (RuntimeEnabledFeatures::overlayScrollbarsEnabled())
+        return NSScrollerStyleOverlay;
+
+    // The ScrollerStyleObserver will update g_scrollerStyle at init and when needed.
+    // This function is hot.
+    // http://crbug.com/303205
+    static ScrollerStyleObserver* scrollerStyleObserver = [[ScrollerStyleObserver alloc] init];
+    (void)scrollerStyleObserver;
+    return g_scrollerStyle;
+}
+
+}