Transmit a X-Chrome-UMA-Enabled bit to Google domains from clients that have UMA enabled.

chrome/browser/net/chrome_network_delegate.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_NET_CHROME_NETWORK_DELEGATE_H_
 #define CHROME_BROWSER_NET_CHROME_NETWORK_DELEGATE_H_
 #pragma once
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
 #include "net/base/network_delegate.h"
 
 class CookieSettings;
 class ExtensionEventRouterForwarder;
 class ExtensionInfoMap;
 class PrefService;
 template<class T> class PrefMember;
 
 typedef PrefMember<bool> BooleanPrefMember;
 
 namespace policy {
 class URLBlacklistManager;
 }
 
 // ChromeNetworkDelegate is the central point from within the chrome code to
 // add hooks into the network stack.
 class ChromeNetworkDelegate : public net::NetworkDelegate {
  public:
+  // Whether or not Incognito-mode is on for the browser window that owns a
+  // ChromeNetworkDelegate. This determines if we are permitted to transmit

[battre, 2012/04/23 10:49:55]

This comment is not quite accurate: a) you cannot switch the incognito mode on
and off for a window, b) the system_request_context is not owned by a window.

My proposed formulation:
Whether or not the URLRequestContext associated with the ChromeNetworkDelegate
belongs to an incognito profile. This determines ...

[SteveT, 2012/04/27 00:05:07]

All of this has been removed.

+  // certain bits of information or not.
+  enum IncognitoState {
+    INCOGNITO_ON,
+    INCOGNITO_OFF,
+  };
+
   // If |profile| is NULL, events will be broadcasted to all profiles,
   // otherwise they will only be sent to the specified profile.
-  // |enable_referrers| should be initialized on the UI thread (see below)
-  // beforehand. This object's owner is responsible for cleaning it up at
-  // shutdown. If |cookie_settings| is NULL, all cookies are enabled,
-  // otherwise, the settings are enforced on all observed network requests.
+  // |enable_referrers| and |enable_metrics| should be initialized on the UI
+  // thread (see below) beforehand, though |enable_metrics| is optional and
+  // will be treated as false if not passed in. This object's owner is
+  // responsible for cleaning it up at shutdown. If |cookie_settings| is NULL,
+  // all cookies are enabled, otherwise, the settings are enforced on all
+  // observed network requests.
   ChromeNetworkDelegate(
       ExtensionEventRouterForwarder* event_router,
       ExtensionInfoMap* extension_info_map,
       const policy::URLBlacklistManager* url_blacklist_manager,
       void* profile,
       CookieSettings* cookie_settings,
-      BooleanPrefMember* enable_referrers);
+      IncognitoState incognito_state,
+      BooleanPrefMember* enable_referrers,
+      BooleanPrefMember* enable_metrics);
   virtual ~ChromeNetworkDelegate();
 
-  // Binds |enable_referrers| to |pref_service| and moves it to the IO thread.
-  // This method should be called on the UI thread.
+  // Binds |enable_referrers| to the kReferrersEnabled pref in |pref_service|
+  // and moves it to the IO thread. This method should be called on the UI
+  // thread.
   static void InitializeReferrersEnabled(BooleanPrefMember* enable_referrers,
                                          PrefService* pref_service);
 
+  // Binds |enable_referrers| to the kMetricsReportingEnabled pref in
+  // |pref_service| and moves it to the IO thread. This method should be called
+  // on the UI thread.
+  static void InitializeMetricsEnabled(BooleanPrefMember* enable_metrics,
+                                       PrefService* pref_service);
+
  private:
   // NetworkDelegate implementation.
   virtual int OnBeforeURLRequest(net::URLRequest* request,
                                  const net::CompletionCallback& callback,
                                  GURL* new_url) OVERRIDE;
   virtual int OnBeforeSendHeaders(net::URLRequest* request,
                                   const net::CompletionCallback& callback,
                                   net::HttpRequestHeaders* headers) OVERRIDE;
   virtual void OnSendHeaders(net::URLRequest* request,
                              const net::HttpRequestHeaders& headers) OVERRIDE;
@@ skipping 16 matching lines @@
       net::URLRequest* request,
       const net::AuthChallengeInfo& auth_info,
       const AuthCallback& callback,
       net::AuthCredentials* credentials) OVERRIDE;
   virtual bool CanGetCookies(const net::URLRequest* request,
                              const net::CookieList& cookie_list) OVERRIDE;
   virtual bool CanSetCookie(const net::URLRequest* request,
                             const std::string& cookie_line,
                             net::CookieOptions* options) OVERRIDE;
 
+  // Adds Chrome experiment and metrics state as custom headers to |headers|
+  // based on local state and |request|.
+  void AddChromeMetricsStateHeader(net::URLRequest* request,
+                                   net::HttpRequestHeaders* headers);
+
   scoped_refptr<ExtensionEventRouterForwarder> event_router_;
   void* profile_;
   scoped_refptr<CookieSettings> cookie_settings_;
 
+  // The Incognito state of the browser window that uses this delegate. Note

[battre, 2012/04/23 10:49:55]

see above regarding "the browser window"

[SteveT, 2012/04/27 00:05:07]

All of this has been removed.

+  // that this field is not valid if |profile_| is NULL, so always check
+  // profile_ first before this field.
+  IncognitoState incognito_state_;
+
   scoped_refptr<ExtensionInfoMap> extension_info_map_;
 
   // Weak, owned by our owner.
   BooleanPrefMember* enable_referrers_;
 
   // Weak, owned by our owner.
+  BooleanPrefMember* enable_metrics_;
+
+  // Weak, owned by our owner.
   const policy::URLBlacklistManager* url_blacklist_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(ChromeNetworkDelegate);
 };
 
 #endif  // CHROME_BROWSER_NET_CHROME_NETWORK_DELEGATE_H_