Use Int64_t for PerfResult.total_bytes_downloaded to avoid overflow.

BUG=706515

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I4e5a3fb4b42c8367abb0e46d8f5383950442e064
Reviewed-on: https://chromium-review.googlesource.com/801050Reviewed-by: Misha Efimov <mef@chromium.org>
Reviewed-by: Andrei Kapishnikov <kapishnikov@chromium.org>
Commit-Queue: Andrei Kapishnikov <kapishnikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520609}

components/cronet/ios/test/cronet_performance_test.mm

@@ -32,7 +32,7 @@ struct PerfResult {
   NSTimeInterval total;
   NSTimeInterval mean;
   NSTimeInterval max;
-  int total_bytes_downloaded;
+  int64_t total_bytes_downloaded;
   int failed_requests;
   int total_requests;
 };