Make scrollAnimatorEnabled work on Mac like it does on other platforms

ui/gfx/animation/animation_mac.mm

+// Copyright (c) 2016 The Chromium Authors. All rights reserved.

[Avi (use Gerrit), 2016/02/10 00:24:54]

no (c)

[bokan, 2016/02/10 16:31:22]

Done.

+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "ui/gfx/animation/animation.h"
+
+#include "base/mac/mac_util.h"
+#include "base/message_loop/message_loop.h"
+
+namespace gfx {
+
+// static
+bool Animation::ScrollAnimationsEnabledBySystem() {
+  // Because of sandboxing, OS settings should only be queried from the browser
+  // process.
+  DCHECK(base::MessageLoopForUI::IsCurrent() ||

[Robert Sesek, 2016/02/09 18:57:45]

Not sure about how useful this DCHCEK is. It would be nice to have a
is-browser-only check, but I don't think that's possible to do correctly without
a layering violation.

[bokan, 2016/02/09 19:00:38]

AFAICT, renderer processes run on a "custom" message loop so it should catch
calls from the renderer process, right? I'll leave it in unless that's wrong.

+      base::MessageLoopForIO::IsCurrent());
+
+  bool enabled = false;
+  id value = nil;
+  if (base::mac::IsOSMountainLionOrLater()) {
+    value = [[NSUserDefaults standardUserDefaults]
+        objectForKey:@"NSScrollAnimationEnabled"];
+  } else {
+    value = [[NSUserDefaults standardUserDefaults]
+        objectForKey:@"AppleScrollAnimationEnabled"];
+  }
+  if (value)
+    enabled = [value boolValue];
+  return enabled;
+}
+
+} // namespace gfx