[CleanUp] Use base::STLSetDifference in place of std::set_difference

ui/base/test/ui_cocoa_test_helper.mm

Index: ui/base/test/ui_cocoa_test_helper.mm
diff --git a/ui/base/test/ui_cocoa_test_helper.mm b/ui/base/test/ui_cocoa_test_helper.mm
index 478aa31edaf4156ad4fe132be1d6e757a73d0bef..a33228f27f222c0f71c7cb01bd3333db8b5e9aec 100644
--- a/ui/base/test/ui_cocoa_test_helper.mm
+++ b/ui/base/test/ui_cocoa_test_helper.mm
@@ -6,6 +6,7 @@
 
 #include "base/debug/debugger.h"
 #include "base/logging.h"
+#include "base/stl_util.h"
 #include "base/test/test_timeouts.h"
 
 @implementation CocoaTestHelperWindow
@@ -179,10 +180,8 @@ std::set<NSWindow*> CocoaTest::ApplicationWindows() {
 
 std::set<NSWindow*> CocoaTest::WindowsLeft() {
   const std::set<NSWindow*> windows(ApplicationWindows());
-  std::set<NSWindow*> windows_left;
-  std::set_difference(windows.begin(), windows.end(),
-                      initial_windows_.begin(), initial_windows_.end(),
-                      std::inserter(windows_left, windows_left.begin()));
+  std::set<NSWindow*> windows_left =
+      base::STLSetDifference<std::set<NSWindow*> >(windows, initial_windows_);
   return windows_left;
 }