[cronet] Expose API to set experimental options

components/cronet/ios/cronet_environment.mm

Index: components/cronet/ios/cronet_environment.mm
diff --git a/components/cronet/ios/cronet_environment.mm b/components/cronet/ios/cronet_environment.mm
index 40fb6ac9bedadccaa129f0b50bd2ecff31a0567d..1532d34c173fac973c447117c4e2872deee4008e 100644
--- a/components/cronet/ios/cronet_environment.mm
+++ b/components/cronet/ios/cronet_environment.mm
@@ -221,6 +221,7 @@ CronetEnvironment::CronetEnvironment(const std::string& user_agent,
                                      bool user_agent_partial)
     : http2_enabled_(false),
       quic_enabled_(false),
+      experimental_options_("{}"),

[mef, 2017/03/21 20:44:57]

nit: I don't think this is necessary as it is covered on objC side and empty
string appears to be a valid default.

[lilyhoughton, 2017/03/22 13:40:55]

Done.  Should I have Obj-C default be an empty string as well?

How much should we be thinking about what the use of C++ API without Obj-C will
look like/having sane defaults for that?

[mef, 2017/03/22 19:18:26]

 I think so. It seems that on Android they default to empty string:
https://cs.chromium.org/chromium/src/components/cronet/android/java/src/org/c...
That would be nice to reduce confusion.

       http_cache_(URLRequestContextConfig::HttpCacheType::DISK),
       user_agent_(user_agent),
       user_agent_partial_(user_agent_partial),
@@ -293,6 +294,8 @@ void CronetEnvironment::InitializeOnNetworkThread() {
       cache_path.value();  // Storage path for http cache and cookie storage.
   context_config_builder.user_agent =
       user_agent_;  // User-Agent request header field.
+  context_config_builder.experimental_options =

[mef, 2017/03/21 20:44:58]

I think we also need to do something similar to
CronetURLRequestContextAdapter::GetNetLogInfo() in order to be able to log those
as part of net log :
https://cs.chromium.org/chromium/src/components/cronet/android/cronet_url_req...

Check with xunjieli@, It should be ok to do as followup CL depending on how big
are required changes.

[lilyhoughton, 2017/03/22 13:40:55]

Acknowledged.

+      experimental_options_;  // Set experimental Cronet options.
   context_config_builder.mock_cert_verifier = std::move(
       mock_cert_verifier_);  // MockCertVerifier to use for testing purposes.
   std::unique_ptr<URLRequestContextConfig> config =