Enable smooth scrolling on mac

chrome/browser/tab_contents/render_view_host_delegate_helper.cc

Index: chrome/browser/tab_contents/render_view_host_delegate_helper.cc
diff --git a/chrome/browser/tab_contents/render_view_host_delegate_helper.cc b/chrome/browser/tab_contents/render_view_host_delegate_helper.cc
index 4958bc89a5d8c970bcea7bd231ffac15654843e8..11371de42eeb9343c8dde38924ed3c2f8fc0fd31 100644
--- a/chrome/browser/tab_contents/render_view_host_delegate_helper.cc
+++ b/chrome/browser/tab_contents/render_view_host_delegate_helper.cc
@@ -460,8 +460,20 @@ WebPreferences RenderViewHostDelegateHelper::GetWebkitPrefs(
         prefs->GetBoolean(prefs::kWebKitAllowDisplayingInsecureContent);
     web_prefs.allow_running_insecure_content =
         prefs->GetBoolean(prefs::kWebKitAllowRunningInsecureContent);
-    web_prefs.enable_scroll_animator =
-        command_line.HasSwitch(switches::kEnableSmoothScrolling);
+
+#if defined(OS_MACOSX)
+    bool default_enable_scroll_animator = true;
+#else
+    // On CrOS, the launcher always passes in the --enable flag.
+    bool default_enable_scroll_animator = false;
+#endif
+    web_prefs.enable_scroll_animator = default_enable_scroll_animator;
+    if (command_line.HasSwitch(switches::kEnableSmoothScrolling))
+      web_prefs.enable_scroll_animator = true;
+#if defined(OS_MACOSX)
+    if (command_line.HasSwitch(switches::kDisableMacSmoothScrollingForTesting))
+      web_prefs.enable_scroll_animator = false;
+#endif
 
     // The user stylesheet watcher may not exist in a testing profile.
     if (profile->GetUserStyleSheetWatcher()) {