[cronet] Add mechanism for restarting CronetEnvironment

components/cronet/ios/test/get_stream_engine.mm

Index: components/cronet/ios/test/get_stream_engine.mm
diff --git a/components/cronet/ios/test/get_stream_engine.mm b/components/cronet/ios/test/get_stream_engine.mm
index f73f1c9a232266d112de42549250f2fb417e0b75..6e20db70782e8f46e4acfca17247854e7c93e9c0 100644
--- a/components/cronet/ios/test/get_stream_engine.mm
+++ b/components/cronet/ios/test/get_stream_engine.mm
@@ -8,11 +8,22 @@
 #include "components/cronet/ios/test/start_cronet.h"
 #include "components/grpc_support/test/get_stream_engine.h"
 
+@interface Cronet (ExposedForTesting)
++ (void)shutdownForTesting;
+@end
+
 namespace grpc_support {
 
 stream_engine* GetTestStreamEngine(int port) {
-  cronet::StartCronetIfNecessary(port);
   return [Cronet getGlobalEngine];
 }
 
+void StartTestStreamEngine(int port) {
+  cronet::StartCronet(port);
+}
+
+void ShutdownTestStreamEngine() {
+  [Cronet shutdownForTesting];
+}
+
 }  // namespace grpc_support