Cache results from respondsToSelector.

Source/core/platform/mac/NSScrollerImpDetails.mm

Index: Source/core/platform/mac/NSScrollerImpDetails.mm
diff --git a/Source/core/platform/mac/NSScrollerImpDetails.mm b/Source/core/platform/mac/NSScrollerImpDetails.mm
index a5f09c5d519c0df39a5713c72bc1acc9152a3ce1..464bbc0c4b498fa665be8c0a6913010bd023d5c7 100644
--- a/Source/core/platform/mac/NSScrollerImpDetails.mm
+++ b/Source/core/platform/mac/NSScrollerImpDetails.mm
@@ -48,7 +48,13 @@ bool isScrollbarOverlayAPIAvailable()
 NSScrollerStyle recommendedScrollerStyle() {
     if (RuntimeEnabledFeatures::overlayScrollbarsEnabled())
         return NSScrollerStyleOverlay;
-    if ([NSScroller respondsToSelector:@selector(preferredScrollerStyle)])

[Nico, 2013/10/11 19:48:48]

If this is called from a single thread, just 

static bool respondsToPreferredScrollerStyle = [NSScroller
respondsToSelector:@selector(preferredScrollerStyle)];  // static for speed

would be enough and a tad shorter.

+    bool respondsToSelectorPreferredScrollerStyle;
+    bool shouldInitialize = true;

[Nico, 2013/10/11 19:48:48]

…wait, how does this version work? Isn't this still called every frame? Did you
check in a profiler that the hotspot actually goes away?

+    if (shouldInitialize) {
+        shouldInitialize = false;
+        respondsToSelectorPreferredScrollerStyle = [NSScroller respondsToSelector:@selector(preferredScrollerStyle)];
+    }
+    if (respondsToSelectorPreferredScrollerStyle)
         return [NSScroller preferredScrollerStyle];
     return NSScrollerStyleLegacy;
 }