Use of base::StringPairs appropriately in base

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

BUG=412250

Review URL: https://codereview.chromium.org/1104693003

Cr-Commit-Position: refs/heads/master@{#327129}

AUTHORS

@@ -378,6 +378,7 @@ Paritosh Kumar <paritosh.in@samsung.com>
 Patrasciuc Sorin Cristian <cristian.patrasciuc@gmail.com>
 Patrick Kettner <patrickkettner@gmail.com>
 Patrick Riordan <patrickriordan177@gmail.com>
+Patrik Ackland <patrikackland@gmail.com>
 Paul Adolph <padolph@netflix.com>
 Paul Kehrer <paul.l.kehrer@gmail.com>
 Paul Lind <paul.lind@imgtec.com>

base/sys_info_chromeos.cc

@@ -109,7 +109,7 @@ class ChromeOSVersionInfo {
     // Parse and cache lsb_release key pairs. There should only be a handful
     // of entries so the overhead for this will be small, and it can be
     // useful for debugging.
-    std::vector<std::pair<std::string, std::string> > pairs;
+    base::StringPairs pairs;
     SplitStringIntoKeyValuePairs(lsb_release, '=', '\n', &pairs);
     for (size_t i = 0; i < pairs.size(); ++i) {
       std::string key, value;