win vs2015: fix variable shadowing warnings

net/base/mime_util.cc

Index: net/base/mime_util.cc
diff --git a/net/base/mime_util.cc b/net/base/mime_util.cc
index 7b51b41faf5c112e329effd848907c605b1e1875..889525bb3ed02a3afbf58eb08025f5f9f64d44e9 100644
--- a/net/base/mime_util.cc
+++ b/net/base/mime_util.cc
@@ -1275,7 +1275,6 @@ void GetExtensionsFromHardCodedMappings(
     size_t mappings_len,
     const std::string& leading_mime_type,
     base::hash_set<base::FilePath::StringType>* extensions) {
-  base::FilePath::StringType extension;
   for (size_t i = 0; i < mappings_len; ++i) {
     if (StartsWithASCII(mappings[i].mime_type, leading_mime_type, false)) {
       std::vector<string> this_extensions;