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 ed295c0643a73f298d561cde626476b6da9f8967..94f8bc626333f011facde05a2dee3bb7383e4c74 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',