Provide mutable members of UrlRequestContext via pure-virtual interface

chrome/chrome_browser.gypi

Index: chrome/chrome_browser.gypi
diff --git a/chrome/chrome_browser.gypi b/chrome/chrome_browser.gypi
index 57b4fff33b542359ae53d903074d836ea763156b..6d9c6e0f4832618627b4c1a28cc76e62e08ddc50 100644
--- a/chrome/chrome_browser.gypi
+++ b/chrome/chrome_browser.gypi
@@ -1057,9 +1057,13 @@
         'browser/net/about_protocol_handler.h',
         'browser/net/async_dns_field_trial.cc',
         'browser/net/async_dns_field_trial.h',
+        'browser/net/basic_http_user_agent_settings.cc',
+        'browser/net/basic_http_user_agent_settings.h',
         'browser/net/chrome_cookie_notification_details.h',
         'browser/net/chrome_fraudulent_certificate_reporter.cc',
         'browser/net/chrome_fraudulent_certificate_reporter.h',
+        'browser/net/chrome_http_user_agent_settings.cc',
+        'browser/net/chrome_http_user_agent_settings.h',
         'browser/net/chrome_net_log.cc',
         'browser/net/chrome_net_log.h',
         'browser/net/chrome_network_delegate.cc',