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.cc b/quic/qbone/qbone_packet_processor.cc
index 9475e27..ef78efa 100644
--- a/quic/qbone/qbone_packet_processor.cc
+++ b/quic/qbone/qbone_packet_processor.cc
@@ -60,7 +60,8 @@
   return ProcessingResult::OK;
 }
 
-void QbonePacketProcessor::ProcessPacket(string* packet, Direction direction) {
+void QbonePacketProcessor::ProcessPacket(std::string* packet,
+                                         Direction direction) {
   if (QUIC_PREDICT_FALSE(!IsValid())) {
     QUIC_BUG << "QuicPacketProcessor is invoked in an invalid state.";
     stats_->OnPacketDroppedSilently(direction);
@@ -106,7 +107,7 @@
 }
 
 QbonePacketProcessor::ProcessingResult
-QbonePacketProcessor::ProcessIPv6HeaderAndFilter(string* packet,
+QbonePacketProcessor::ProcessIPv6HeaderAndFilter(std::string* packet,
                                                  Direction direction,
                                                  uint8_t* transport_protocol,
                                                  char** transport_data,
@@ -154,7 +155,7 @@
 }
 
 QbonePacketProcessor::ProcessingResult QbonePacketProcessor::ProcessIPv6Header(
-    string* packet,
+    std::string* packet,
     Direction direction,
     uint8_t* transport_protocol,
     char** transport_data,