[Cronet] Fix HistogramManager.HistogramBucketFields crashes on Win7 Tests

Bug: 812268
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I15a0d9f4e3a0c16d9a63e82ebb10943e082e1835
Reviewed-on: https://chromium-review.googlesource.com/999331Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548845}

components/cronet/histogram_manager.cc

@@ -49,7 +49,7 @@ bool HistogramManager::GetDeltas(std::vector<uint8_t>* data) {
         &histogram_snapshot_manager_);
     int32_t data_size = uma_proto_.ByteSize();
     data->resize(data_size);
-    if (uma_proto_.SerializeToArray(&(*data)[0], data_size))
+    if (data_size == 0 || uma_proto_.SerializeToArray(data->data(), data_size))
       return true;
   }
   data->clear();