Migrate StringPiece::CopyToString to string::assign.

net/tools/quic/quic_http_response_cache.h

Index: net/tools/quic/quic_http_response_cache.h
diff --git a/net/tools/quic/quic_http_response_cache.h b/net/tools/quic/quic_http_response_cache.h
index b77b80b628adf0ad06765fd9a94b0b238a81aa84..5c8f6ea779887cdb693ed196a6d8c848e826c30f 100644
--- a/net/tools/quic/quic_http_response_cache.h
+++ b/net/tools/quic/quic_http_response_cache.h
@@ -65,7 +65,9 @@ class QuicHttpResponseCache {
     void set_trailers(SpdyHeaderBlock trailers) {
       trailers_ = std::move(trailers);
     }
-    void set_body(QuicStringPiece body) { body.CopyToString(&body_); }
+    void set_body(QuicStringPiece body) {
+      body_.assign(body.data(), body.size());
+    }
 
    private:
     SpecialResponseType response_type_;