Fixed crashes in CrNetEnvironment::ConfigureSdchOnNetworkThread()

components/cronet/ios/Cronet.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import <Foundation/Foundation.h>
 
 #include "cronet_c_for_grpc.h"
 
 // Interface for installing Cronet.
 CRONET_EXPORT
@@ skipping 14 matching lines @@
 // |userAgent| is expected to be of the form Product/Version.
 // Example: Foo/3.0.0.0
 //
 // This method only has any effect before |start| is called.
 + (void)setPartialUserAgent:(NSString*)userAgent;
 
 // Sets SSLKEYLogFileName to export SSL key for Wireshark decryption of packet
 // captures. This method only has any effect before |start| is called.
 + (void)setSslKeyLogFileName:(NSString*)sslKeyLogFileName;
 
-// Starts CronetEngine.
+// Starts CronetEngine. It is recommended to call this method on the application
+// main thread. If the method is called on any thread other than the main one,
+// the method will internally try to execute synchronously using the main GCD
+// queue. Please make sure that the main thread is not blocked by a job
+// that calls this method; otherwise, a deadlock can occur.
 + (void)start;
 
 // Starts net-internals logging to a file named |fileName| in the application
 // temporary directory. |fileName| must not be empty. Log level is determined
 // by |logBytes| - if YES then LOG_ALL otherwise LOG_ALL_BUT_BYTES. If the file
 // exists it is truncated before starting. If actively logging the call is
 // ignored.
 + (void)startNetLogToFile:(NSString*)fileName logBytes:(BOOL)logBytes;
 
 // Stop net-internals logging and flush file to disk. If a logging session is
 // not in progress this call is ignored.
 + (void)stopNetLog;
 
 // Returns the full user-agent that the stack uses.
 // This is the exact string servers will see.
 + (NSString*)getUserAgent;
 
 // Get a pointer to global instance of cronet_engine for GRPC C API.
 + (cronet_engine*)getGlobalEngine;
 
 @end