Add unittests for certificate and file parsing.

net/tools/transport_security_state_generator/spki_hash.cc

Index: net/tools/transport_security_state_generator/spki_hash.cc
diff --git a/net/tools/transport_security_state_generator/spki_hash.cc b/net/tools/transport_security_state_generator/spki_hash.cc
index a6986fc975bee6dafb57ce8b07e78dc6e82de90b..e7914c3227dd7fed38768da5ce971a1fe43ec8dd 100644
--- a/net/tools/transport_security_state_generator/spki_hash.cc
+++ b/net/tools/transport_security_state_generator/spki_hash.cc
@@ -19,8 +19,8 @@ SPKIHash::SPKIHash() {}
 
 SPKIHash::~SPKIHash() {}
 
-bool SPKIHash::FromString(const std::string& hash_string) {
-  std::string base64_string;
+bool SPKIHash::FromString(base::StringPiece hash_string) {
+  base::StringPiece base64_string;
 
   if (!base::StartsWith(hash_string, "sha256/",
                         base::CompareCase::INSENSITIVE_ASCII)) {