LSC: Add std:: qualifications to all references to std::string and std::basic_string.

Adding these qualifications will make google3 C++ more portable, allow the global using std::string declarations in the google3 copy of the C++ standard library to be deleted, and bring google3 C++ in line with how the rest of the world uses C++. This completes the work started in go/lsc-add-std and go/std-type-qualification.

LSC documentation: go/lsc-std-string

Tested:
    tap_presubmit: http://test/OCL:296219821:BASE:296205992:1582238469429:4f5b2b3
    Some tests failed; test failures are believed to be unrelated to this CL
PiperOrigin-RevId: 296825017
Change-Id: I2b41071f7f9cc790308a6c95270659dcd57e3a27
diff --git a/quic/qbone/qbone_packet_processor_test_tools.cc b/quic/qbone/qbone_packet_processor_test_tools.cc
index 9f9b623..97655e0 100644
--- a/quic/qbone/qbone_packet_processor_test_tools.cc
+++ b/quic/qbone/qbone_packet_processor_test_tools.cc
@@ -8,7 +8,7 @@
 
 namespace quic {
 
-string PrependIPv6HeaderForTest(const string& body, int hops) {
+std::string PrependIPv6HeaderForTest(const std::string& body, int hops) {
   ip6_hdr header;
   memset(&header, 0, sizeof(header));
 
@@ -19,7 +19,7 @@
   header.ip6_src = in6addr_loopback;
   header.ip6_dst = in6addr_loopback;
 
-  string packet(sizeof(header) + body.size(), '\0');
+  std::string packet(sizeof(header) + body.size(), '\0');
   memcpy(&packet[0], &header, sizeof(header));
   memcpy(&packet[sizeof(header)], body.data(), body.size());
   return packet;