Replace QuicString with std::string, pass 1

This replaces QuicString with std::string in all of the "QUIC proper".  I will
delete QuicString once all code using it is gone.

gfe-relnote: n/a (no functional change)
PiperOrigin-RevId: 237872023
Change-Id: I82de62c9855516b15039734d05155917e68ff4ee
diff --git a/quic/core/crypto/quic_hkdf_test.cc b/quic/core/crypto/quic_hkdf_test.cc
index f8c53c6..fbf2860 100644
--- a/quic/core/crypto/quic_hkdf_test.cc
+++ b/quic/core/crypto/quic_hkdf_test.cc
@@ -70,10 +70,10 @@
     const HKDFInput& test(kHKDFInputs[i]);
     SCOPED_TRACE(i);
 
-    const QuicString key = QuicTextUtils::HexDecode(test.key_hex);
-    const QuicString salt = QuicTextUtils::HexDecode(test.salt_hex);
-    const QuicString info = QuicTextUtils::HexDecode(test.info_hex);
-    const QuicString expected = QuicTextUtils::HexDecode(test.output_hex);
+    const std::string key = QuicTextUtils::HexDecode(test.key_hex);
+    const std::string salt = QuicTextUtils::HexDecode(test.salt_hex);
+    const std::string info = QuicTextUtils::HexDecode(test.info_hex);
+    const std::string expected = QuicTextUtils::HexDecode(test.output_hex);
 
     // We set the key_length to the length of the expected output and then take
     // the result from the first key, which is the client write key.