Add Accept-Encoding: br for secure data reduction proxy requests.

components/data_reduction_proxy/core/browser/data_reduction_proxy_network_delegate.cc

Index: components/data_reduction_proxy/core/browser/data_reduction_proxy_network_delegate.cc
diff --git a/components/data_reduction_proxy/core/browser/data_reduction_proxy_network_delegate.cc b/components/data_reduction_proxy/core/browser/data_reduction_proxy_network_delegate.cc
index 50a7a09b9e340d19941f6db4775ee197343cbf61..5dd973c2854446fc7f7809b42156f768b9f38dd3 100644
--- a/components/data_reduction_proxy/core/browser/data_reduction_proxy_network_delegate.cc
+++ b/components/data_reduction_proxy/core/browser/data_reduction_proxy_network_delegate.cc
@@ -225,6 +225,12 @@ void DataReductionProxyNetworkDelegate::OnBeforeSendHeadersInternal(
 
   if (data_reduction_proxy_request_options_) {
     data_reduction_proxy_request_options_->AddRequestHeader(headers);
+    // Data Reduction Proxy handles Accept-Encoding: Brotli correctly, but don't
+    // trust intermediate proxies.
+    if (proxy_info.is_https() || proxy_info.is_quic()) {
+      data_reduction_proxy_request_options_->AddBrotliAcceptEncoding(*request,
+                                                                     headers);
+    }
   }
 }