Fix variable shadowing warning in CRLSet on VS2015

d:\src\cr3\src\net\cert\crl_set.cc(53): error C2220: warning treated as error - no 'object' file generated
d:\src\cr3\src\net\cert\crl_set.cc(53): warning C4456: declaration of 'i' hides previous local declaration
d:\src\cr3\src\net\cert\crl_set.cc(47): note: see declaration of 'i'

R=mattm@chromium.org
BUG=440500

Review URL: https://codereview.chromium.org/882833002

Cr-Commit-Position: refs/heads/master@{#313410}

net/cert/crl_set.cc

@@ -44,11 +44,11 @@ CRLSet::Result CRLSet::CheckSerial(
   while (serial.size() > 1 && serial[0] == 0x00)
     serial.remove_prefix(1);
 
-  base::hash_map<std::string, size_t>::const_iterator i =
+  base::hash_map<std::string, size_t>::const_iterator crl_index =
       crls_index_by_issuer_.find(issuer_spki_hash.as_string());
-  if (i == crls_index_by_issuer_.end())
+  if (crl_index == crls_index_by_issuer_.end())
     return UNKNOWN;
-  const std::vector<std::string>& serials = crls_[i->second].second;
+  const std::vector<std::string>& serials = crls_[crl_index->second].second;
 
   for (std::vector<std::string>::const_iterator i = serials.begin();
        i != serials.end(); ++i) {