[cronet] Expose API to set experimental options

components/cronet/ios/test/start_cronet.mm

 // 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 <Cronet/Cronet.h>
 #import <Foundation/Foundation.h>
 
 #include "components/cronet/ios/test/start_cronet.h"
 
 #include "base/strings/stringprintf.h"
 #include "base/strings/sys_string_conversions.h"
 #include "components/grpc_support/test/quic_test_server.h"
 
 namespace cronet {
 
 void StartCronetIfNecessary(int port) {
   static bool initialized = false;
   if (!initialized) {
     initialized = true;
     [Cronet setUserAgent:@"CronetTest/1.0.0.0" partial:NO];
     [Cronet setHttp2Enabled:true];
     [Cronet setQuicEnabled:true];
     [Cronet setAcceptLanguages:@"en-US,en"];
-    [Cronet setSslKeyLogFileName:@"SSLKEYLOGFILE"];
+    [Cronet
+        setExperimentalOptions:
+            [NSString
+                stringWithFormat:@"%@%@%@", @"{\"ssl_key_log_file\":\"",

[mef, 2017/04/06 18:58:26]

Can this be simplified to have first and third strings (constants) as part of
format?

+                                 [Cronet getNetLogPathForFile:@"SSLKEYLOGFILE"],
+                                 @"\"}"]];
     [Cronet addQuicHint:@"test.example.com" port:443 altPort:443];
     [Cronet enableTestCertVerifierForTesting];
     [Cronet start];
   }
   NSString* rules = base::SysUTF8ToNSString(
       base::StringPrintf("MAP test.example.com 127.0.0.1:%d,"
                          "MAP notfound.example.com ~NOTFOUND",
                          port));
   [Cronet setHostResolverRulesForTesting:rules];
 }
 
 }  // namespace cronet