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_control_stream.cc b/quic/qbone/qbone_control_stream.cc
index 225d02f..31a5070 100644
--- a/quic/qbone/qbone_control_stream.cc
+++ b/quic/qbone/qbone_control_stream.cc
@@ -38,7 +38,7 @@
     if (buffer_.size() < pending_message_size_) {
       return;
     }
-    string tmp = buffer_.substr(0, pending_message_size_);
+    std::string tmp = buffer_.substr(0, pending_message_size_);
     buffer_.erase(0, pending_message_size_);
     pending_message_size_ = 0;
     OnMessage(tmp);
@@ -46,7 +46,7 @@
 }
 
 bool QboneControlStreamBase::SendMessage(const proto2::Message& proto) {
-  string tmp;
+  std::string tmp;
   if (!proto.SerializeToString(&tmp)) {
     QUIC_BUG << "Failed to serialize QboneControlRequest";
     return false;