win: avoid warning on return of base::RandGenerator

util/net/http_multipart_builder.cc

Index: util/net/http_multipart_builder.cc
diff --git a/util/net/http_multipart_builder.cc b/util/net/http_multipart_builder.cc
index 3f66658c46ee54e54e123f5833d480b35396ac6c..29b00af0c1c0b217bc751e99da789d15d7214a4e 100644
--- a/util/net/http_multipart_builder.cc
+++ b/util/net/http_multipart_builder.cc
@@ -46,7 +46,7 @@ std::string GenerateBoundaryString() {
   for (int index = 0; index < 32; ++index) {
     const char kCharacters[] =
         "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
-    int random_value = base::RandGenerator(strlen(kCharacters));
+    int random_value = base::RandInt(0, strlen(kCharacters) - 1);
     boundary_string += kCharacters[random_value];
   }
   boundary_string += "---";