Adds feature disabling referrers by default

This CL adds a new feature, kNoReferrers, to subsume the functionality
of the --no-referrers switch.  When the feature is *true*, the
kEnableReferrers pref will have its default value set to *false*.  This
is the current effect of setting the --no-referrers switch.

A separate CL, crrev.com/c/1808144, is in progress to remove the switch
itself.

This is one of a series of changes adding
functionality to run a Finch experiment exploring different
levels of referrer policy and caps.

R=creis, mmenke

depending on the value of the feature.

Test: Added browser test to confirm that the pref is set correctly
Bug: 1004486
Change-Id: If250cf4f29523328e540aa23672436071fbbb10a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1806080
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699349}

chrome/browser/net/system_network_context_manager.cc

@@ -476,6 +476,9 @@ SystemNetworkContextManager::SystemNetworkContextManager(
   pref_change_registrar_.Add(prefs::kKerberosEnabled, auth_pref_callback);
 #endif  // defined(OS_CHROMEOS)
 
+  local_state_->SetDefaultPrefValue(
+      prefs::kEnableReferrers,
+      base::Value(!base::FeatureList::IsEnabled(features::kNoReferrers)));
   enable_referrers_.Init(
       prefs::kEnableReferrers, local_state_,
       base::BindRepeating(&SystemNetworkContextManager::UpdateReferrersEnabled,

chrome/browser/net/system_network_context_manager_browsertest.cc

@@ -20,7 +20,6 @@
 #include "chrome/test/base/in_process_browser_test.h"
 #include "components/prefs/pref_service.h"
 #include "components/version_info/version_info.h"
-#include "content/public/common/content_switches.h"
 #include "content/public/common/user_agent.h"
 #include "services/network/public/cpp/network_service_buildflags.h"
 #include "services/network/public/mojom/network_context.mojom.h"
@@ -295,6 +294,36 @@ INSTANTIATE_TEST_SUITE_P(,
                          SystemNetworkContextManagerStubResolverBrowsertest,
                          ::testing::Bool());
 
+class SystemNetworkContextManagerReferrersFeatureBrowsertest
+    : public SystemNetworkContextManagerBrowsertest,
+      public testing::WithParamInterface<bool> {
+ public:
+  SystemNetworkContextManagerReferrersFeatureBrowsertest() {
+    scoped_feature_list_.InitWithFeatureState(features::kNoReferrers,
+                                              GetParam());
+  }
+  ~SystemNetworkContextManagerReferrersFeatureBrowsertest() override {}
+
+  void SetUpOnMainThread() override {}
+
+ private:
+  base::test::ScopedFeatureList scoped_feature_list_;
+};
+
+// Tests that toggling the kNoReferrers feature correctly changes the default
+// value of the kEnableReferrers pref.
+IN_PROC_BROWSER_TEST_P(SystemNetworkContextManagerReferrersFeatureBrowsertest,
+                       TestDefaultReferrerReflectsFeatureValue) {
+  ASSERT_TRUE(!!g_browser_process);
+  PrefService* local_state = g_browser_process->local_state();
+  ASSERT_TRUE(!!local_state);
+  EXPECT_NE(local_state->GetBoolean(prefs::kEnableReferrers), GetParam());
+}
+
+INSTANTIATE_TEST_SUITE_P(,
+                         SystemNetworkContextManagerReferrersFeatureBrowsertest,
+                         ::testing::Bool());
+
 class SystemNetworkContextManagerFreezeQUICUaBrowsertest
     : public SystemNetworkContextManagerBrowsertest,
       public testing::WithParamInterface<bool> {

chrome/common/chrome_features.cc

@@ -513,6 +513,11 @@ const base::Feature kNewNetErrorPageUI{"NewNetErrorPageUI",
                                        base::FEATURE_ENABLED_BY_DEFAULT};
 #endif  // OS_ANDROID
 
+// When kNoReferrers is enabled, most HTTP requests will provide empty
+// referrers instead of their ordinary behavior.
+const base::Feature kNoReferrers{"NoReferrers",
+                                 base::FEATURE_DISABLED_BY_DEFAULT};
+
 #if defined(OS_POSIX)
 // Enables NTLMv2, which implicitly disables NTLMv1.
 const base::Feature kNtlmV2Enabled{"NtlmV2Enabled",

chrome/common/chrome_features.h

@@ -331,6 +331,8 @@ extern const base::Feature kNativeNotifications;
 COMPONENT_EXPORT(CHROME_FEATURES) extern const base::Feature kNewNetErrorPageUI;
 #endif
 
+COMPONENT_EXPORT(CHROME_FEATURES) extern const base::Feature kNoReferrers;
+
 #if defined(OS_POSIX)
 COMPONENT_EXPORT(CHROME_FEATURES) extern const base::Feature kNtlmV2Enabled;
 #endif