Use of base::StringPairs appropriately in base

Using  base::StringPairs in base folder
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/605613003

Cr-Commit-Position: refs/heads/master@{#296647}

base/strings/string_split_unittest.cc

@@ -31,7 +31,7 @@ void SplitString(const std::wstring& str,
 
 class SplitStringIntoKeyValuePairsTest : public testing::Test {
  protected:
-  std::vector<std::pair<std::string, std::string> > kv_pairs;
+  base::StringPairs kv_pairs;
 };
 
 TEST_F(SplitStringIntoKeyValuePairsTest, EmptyString) {