[cronet] Add member for QUIC user agent ID, with getter and setter.

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 62c8407aa863134ef03416e4a4c95733e2582e29..d122171656a92804c5c1fa05f26397e0b79d3468 100644
--- a/components/cronet/ios/cronet_environment.mm
+++ b/components/cronet/ios/cronet_environment.mm
@@ -270,8 +270,10 @@ void CronetEnvironment::InitializeOnNetworkThread() {
   cache_path = cache_path.Append(FILE_PATH_LITERAL("cronet"));
 
   std::unique_ptr<URLRequestContextConfig> config(new URLRequestContextConfig(
-      quic_enabled_,                  // Enable QUIC.
-      "",                             // QUIC User Agent ID.
+      quic_enabled_,  // Enable QUIC.
+      quic_enabled_ && quic_user_agent_id_.empty()
+          ? getDefaultQuicUserAgentId()
+          : quic_user_agent_id_,      // QUIC User Agent ID.
       http2_enabled_,                 // Enable SPDY.
       false,                          // Enable SDCH
       URLRequestContextConfig::DISK,  // Type of http cache.
@@ -358,4 +360,10 @@ void CronetEnvironment::SetHostResolverRulesOnNetworkThread(
   event->Signal();
 }
 
+std::string CronetEnvironment::getDefaultQuicUserAgentId() const {
+  return base::SysNSStringToUTF8([[NSBundle mainBundle]
+             objectForInfoDictionaryKey:@"CFBundleDisplayName"]) +
+         " Cronet/" + CRONET_VERSION;
+}
+
 }  // namespace cronet