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/platform/api/quic_stack_trace.h b/quic/platform/api/quic_stack_trace.h
index 1a1a10c..64411f0 100644
--- a/quic/platform/api/quic_stack_trace.h
+++ b/quic/platform/api/quic_stack_trace.h
@@ -10,7 +10,7 @@
 
 namespace quic {
 
-inline QuicString QuicStackTrace() {
+inline std::string QuicStackTrace() {
   return QuicStackTraceImpl();
 }