Add brotli content-encoding filter.

net/http/http_transaction_test_util.cc

Index: net/http/http_transaction_test_util.cc
diff --git a/net/http/http_transaction_test_util.cc b/net/http/http_transaction_test_util.cc
index 6bd8230c46363c106a347badef77bfe6a7f8c7e1..68ca21955bde1df8aa0a6bf383b56ba337a210a9 100644
--- a/net/http/http_transaction_test_util.cc
+++ b/net/http/http_transaction_test_util.cc
@@ -500,8 +500,8 @@ MockNetworkLayer::MockNetworkLayer()
       done_reading_called_(false),
       stop_caching_called_(false),
       last_create_transaction_priority_(DEFAULT_PRIORITY),
-      clock_(nullptr) {
-}
+      clock_(nullptr),
+      session_(nullptr) {}
 
 MockNetworkLayer::~MockNetworkLayer() {}
 
@@ -529,7 +529,7 @@ HttpCache* MockNetworkLayer::GetCache() {
 }
 
 HttpNetworkSession* MockNetworkLayer::GetSession() {
-  return NULL;
+  return session_;
 }
 
 void MockNetworkLayer::SetClock(base::Clock* clock) {