[Cronet] Have Cronet set the QUIC user agent id to a reasonable default on iOS.

Setting the QUIC user agent ID to a non-default value should be done using the experimental options API.

Bug: 670686
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I79bdbde1b1f8be0af3eff7d8d6080974e322b33c
Reviewed-on: https://chromium-review.googlesource.com/822811
Commit-Queue: Lily Houghton <lilyhoughton@chromium.org>
Reviewed-by: Misha Efimov <mef@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525337}

components/cronet/ios/cronet_environment.h

@@ -79,10 +79,6 @@ class CronetEnvironment {
   bool quic_enabled() const { return quic_enabled_; }
   bool brotli_enabled() const { return brotli_enabled_; }
 
-  void set_quic_user_agent_id(const std::string& quic_user_agent_id) {
-    quic_user_agent_id_ = quic_user_agent_id;
-  }
-
   void set_accept_language(const std::string& accept_language) {
     accept_language_ = accept_language;
   }
@@ -174,7 +170,6 @@ class CronetEnvironment {
   bool http2_enabled_;
   bool quic_enabled_;
   bool brotli_enabled_;
-  std::string quic_user_agent_id_;
   std::string accept_language_;
   std::string experimental_options_;
   // Effective experimental options. Kept for NetLog.

components/cronet/ios/cronet_environment.mm

@@ -336,8 +336,10 @@ void CronetEnvironment::InitializeOnNetworkThread() {
 
   URLRequestContextConfigBuilder context_config_builder;
   context_config_builder.enable_quic = quic_enabled_;   // Enable QUIC.
+  context_config_builder.quic_user_agent_id =
+      getDefaultQuicUserAgentId();                      // QUIC User Agent ID.
   context_config_builder.enable_spdy = http2_enabled_;  // Enable HTTP/2.
-  context_config_builder.http_cache = http_cache_;      // Set HTTP cache
+  context_config_builder.http_cache = http_cache_;      // Set HTTP cache.
   context_config_builder.storage_path =
       storage_path.value();  // Storage path for http cache and prefs storage.
   context_config_builder.user_agent =