Clarify wording around blocking 3rd party cookies and add lab to block reading of 3rd party cookies

chrome/browser/about_flags.cc

 // Copyright (c) 2010 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.
 
 #include "chrome/browser/about_flags.h"
 
 #include <algorithm>
 #include <iterator>
 #include <map>
 #include <set>
@@ skipping 255 matching lines @@
     kOsWin,
     MULTI_VALUE_TYPE(kInstantChoices)
   },
   {
     "instant-autocomplete-immediately",  // FLAGS:RECORD_UMA
     IDS_FLAGS_INSTANT_AUTOCOMPLETE_IMMEDIATELY_NAME,
     IDS_FLAGS_INSTANT_AUTOCOMPLETE_IMMEDIATELY_DESCRIPTION,
     kOsWin | kOsLinux,
     SINGLE_VALUE_TYPE(switches::kInstantAutocompleteImmediately)
   },
+  {
+    "block-reading-third-party-cookies",
+    IDS_FLAGS_BLOCK_ALL_THIRD_PARTY_COOKIES_NAME,
+    IDS_FLAGS_BLOCK_ALL_THIRD_PARTY_COOKIES_DESCRIPTION,
+    kOsAll,
+    SINGLE_VALUE_TYPE(switches::kBlockReadingThirdPartyCookies)
+  },
 };
 
 const Experiment* experiments = kExperiments;
 size_t num_experiments = arraysize(kExperiments);
 
 // Stores and encapsulates the little state that about:flags has.
 class FlagsState {
  public:
   FlagsState() : needs_restart_(false) {}
   void ConvertFlagsToSwitches(PrefService* prefs, CommandLine* command_line);
@@ skipping 397 matching lines @@
 }
 
 const Experiment* GetExperiments(size_t* count) {
   *count = num_experiments;
   return experiments;
 }
 
 }  // namespace testing
 
 }  // namespace about_flags