Use base::StringPairs where appropriate from src/extensions

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/594133003

Cr-Commit-Position: refs/heads/master@{#296369}

extensions/common/permissions/permission_message_util.cc

@@ -5,6 +5,7 @@
 #include "extensions/common/permissions/permission_message_util.h"
 
 #include "base/strings/string_number_conversions.h"
+#include "base/strings/string_split.h"
 #include "base/strings/utf_string_conversions.h"
 #include "extensions/common/permissions/permission_message.h"
 #include "extensions/common/permissions/permission_set.h"