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_stream.h b/quic/qbone/qbone_stream.h
index 6a86e4d..c2fb20a 100644
--- a/quic/qbone/qbone_stream.h
+++ b/quic/qbone/qbone_stream.h
@@ -47,7 +47,7 @@
   void OnDataAvailable() override;
 
  private:
-  string buffer_;
+  std::string buffer_;
   QboneSessionBase* session_;
 };