Enable positional parameters for base::vsnprintf and base::vswprintf on Windows.

base/stringprintf_unittest.cc

Index: base/stringprintf_unittest.cc
diff --git a/base/stringprintf_unittest.cc b/base/stringprintf_unittest.cc
index ffb9c779ac027951579bfddaf2f0ab4cb1c0e1c9..1fc7f82e63d115cba619962bdfcffaaaa807060a 100644
--- a/base/stringprintf_unittest.cc
+++ b/base/stringprintf_unittest.cc
@@ -149,4 +149,17 @@ TEST(StringPrintfTest, Invalid) {
 }
 #endif
 
+// Test that the positional parameters work.
+TEST(StringPrintfTest, PositionalParameters) {
+  std::string out;
+  SStringPrintf(&out, "%1$s %1$s", "test");
+  EXPECT_STREQ("test test", out.c_str());
+
+#if defined(OS_WIN)
+  std::wstring wout;
+  SStringPrintf(&wout, L"%1$ls %1$ls", L"test");
+  EXPECT_STREQ(L"test test", wout.c_str());
+#endif
+}
+
 }  // namespace base