Fix printing of downloaded bytes in Cronet perf tests.

[delegate_ totalBytesReceivedPerTask][task] returns a NSNumber
that should be converted to integer for proper printing.

BUG=706515

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I57c7ecea727a13ef5a79a205dee1c134dfe93eea
Reviewed-on: https://chromium-review.googlesource.com/797952Reviewed-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@{#520600}

components/cronet/ios/test/cronet_performance_test.mm

@@ -217,7 +217,8 @@ TEST_P(PerfTest, NSURLSessionReceivesImageLoop) {
     }
 
     if (kUseExternalUrl && success && !first_log) {
-      LOG(INFO) << "Downloaded " << [delegate_ totalBytesReceivedPerTask][task]
+      LOG(INFO) << "Downloaded "
+                << [[delegate_ totalBytesReceivedPerTask][task] intValue]
                 << " bytes on first iteration.";
       first_log = true;
     }