Use base::StringPairs where appropriate from src/google_apis

Becuase base/strings/string_split.h defines:
typedef std::vector<std::pair<std::string, std::string> > StringPairs;

BUG=412250

Review URL: https://codereview.chromium.org/591413002

Cr-Commit-Position: refs/heads/master@{#296888}

google_apis/gaia/gaia_auth_fetcher.cc

@@ -867,10 +867,10 @@ void GaiaAuthFetcher::OnGetUserInfoFetched(
     const net::URLRequestStatus& status,
     int response_code) {
   if (status.is_success() && response_code == net::HTTP_OK) {
-    std::vector<std::pair<std::string, std::string> > tokens;
+    base::StringPairs tokens;
     UserInfoMap matches;
     base::SplitStringIntoKeyValuePairs(data, '=', '\n', &tokens);
-    std::vector<std::pair<std::string, std::string> >::iterator i;
+    base::StringPairs::iterator i;
     for (i = tokens.begin(); i != tokens.end(); ++i) {
       matches[i->first] = i->second;
     }