[cronet] Expose API to set experimental options

components/cronet/ios/test/start_cronet.mm

Index: components/cronet/ios/test/start_cronet.mm
diff --git a/components/cronet/ios/test/start_cronet.mm b/components/cronet/ios/test/start_cronet.mm
index 148fe5333f2101a0e5afe1e14d3f768a5b479f54..4a78f14aec008da6441f4bcd8f5e1aefd49f027c 100644
--- a/components/cronet/ios/test/start_cronet.mm
+++ b/components/cronet/ios/test/start_cronet.mm
@@ -21,7 +21,12 @@ void StartCronetIfNecessary(int port) {
     [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];