[Media Router] Ensure MediaRouterEnabled() does not change per-profile.

A runtime change to the kMediaRouterEnabled policy (managed preference) results
in undefined behavior and crashes.  This is a workaround to ensure that
MediaRouterEnabled() returns the same value for the lifetime of a profile.

Fixing the Media Router to be enabled/disabled dynamically is possible, but will
be blocked on extension removal (specifically the MediaRouteProvider mojo
interface).

Tested by manually enabling/disabling the policy while running Chrome and
ensuring no crashes or partially-enabled UI states happen.

Bug: 1076831
Change-Id: I966a58196f92be40aed8771282c27f762102da6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2597752
Commit-Queue: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838705}

chrome/browser/media/router/BUILD.gn

@@ -250,7 +250,12 @@ source_set("unittests") {
       "providers/extension/extension_media_route_provider_proxy_unittest.cc",
       "providers/wired_display/wired_display_media_route_provider_unittest.cc",
     ]
-    deps += [ ":test_support" ]
+    deps += [
+      ":test_support",
+      "//chrome/test:test_support",
+      "//components/sync_preferences:test_support",
+      "//content/test:test_support",
+    ]
   }
 
   if (enable_openscreen) {

chrome/browser/media/router/media_router_feature.cc

@@ -4,8 +4,12 @@
 
 #include "chrome/browser/media/router/media_router_feature.h"
 
+#include <utility>
+
 #include "base/base64.h"
+#include "base/containers/flat_map.h"
 #include "base/feature_list.h"
+#include "base/no_destructor.h"
 #include "base/strings/string_util.h"
 #include "build/build_config.h"
 #include "chrome/browser/profiles/profile.h"
@@ -66,11 +70,22 @@ bool MediaRouterEnabled(content::BrowserContext* context) {
 #endif  // !defined(OFFICIAL_BUILD) && !defined(OS_ANDROID)
 
 #if defined(OS_ANDROID) || BUILDFLAG(ENABLE_EXTENSIONS)
+  static base::NoDestructor<base::flat_map<content::BrowserContext*, bool>>
+      stored_pref_values;
+
+  // If the Media Router was already enabled or disabled for |context|, then it
+  // must remain so.  The Media Router does not support dynamic
+  // enabling/disabling.
+  auto const it = stored_pref_values->find(context);
+  if (it != stored_pref_values->end())
+    return it->second;
+
+  // Check the enterprise policy.
   const PrefService::Preference* pref = GetMediaRouterPref(context);
-  // Only use the pref value if it set from a mandatory policy.
   if (pref->IsManaged() && !pref->IsDefaultValue()) {
-    bool allowed = false;
+    bool allowed;
     CHECK(pref->GetValue()->GetAsBoolean(&allowed));
+    stored_pref_values->insert(std::make_pair(context, allowed));
     return allowed;
   }
 

chrome/browser/media/router/media_router_feature_unittest.cc

@@ -4,11 +4,23 @@
 
 #include "chrome/browser/media/router/media_router_feature.h"
 
+#include <memory>
+
 #include "base/test/scoped_feature_list.h"
+#include "build/build_config.h"
 #include "components/prefs/pref_registry_simple.h"
 #include "components/prefs/testing_pref_service.h"
+#include "extensions/buildflags/buildflags.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
+#if defined(OS_ANDROID) || BUILDFLAG(ENABLE_EXTENSIONS)
+#include "base/values.h"
+#include "chrome/common/pref_names.h"
+#include "chrome/test/base/testing_profile.h"
+#include "components/sync_preferences/testing_pref_service_syncable.h"
+#include "content/public/test/browser_task_environment.h"
+#endif  // defined(OS_ANDROID) || BUILDFLAG(ENABLE_EXTENSIONS)
+
 namespace media_router {
 
 TEST(MediaRouterFeatureTest, GetCastAllowAllIPsPref) {
@@ -42,4 +54,41 @@ TEST(MediaRouterFeatureTest, GetReceiverIdHashToken) {
   EXPECT_EQ(token, GetReceiverIdHashToken(pref_service.get()));
 }
 
+#if defined(OS_ANDROID) || BUILDFLAG(ENABLE_EXTENSIONS)
+class MediaRouterEnabledTest : public ::testing::Test {
+ public:
+  MediaRouterEnabledTest() = default;
+  MediaRouterEnabledTest(const MediaRouterEnabledTest&) = delete;
+  ~MediaRouterEnabledTest() override = default;
+  MediaRouterEnabledTest& operator=(const MediaRouterEnabledTest&) = delete;
+
+ protected:
+  content::BrowserTaskEnvironment test_environment;
+  TestingProfile enabled_profile;
+  TestingProfile disabled_profile;
+};
+
+TEST_F(MediaRouterEnabledTest, TestEnabledByPolicy) {
+  enabled_profile.GetTestingPrefService()->SetManagedPref(
+      ::prefs::kEnableMediaRouter, std::make_unique<base::Value>(true));
+  EXPECT_TRUE(MediaRouterEnabled(&enabled_profile));
+
+  enabled_profile.GetTestingPrefService()->SetManagedPref(
+      ::prefs::kEnableMediaRouter, std::make_unique<base::Value>(false));
+  // Runtime changes are not supported.
+  EXPECT_TRUE(MediaRouterEnabled(&enabled_profile));
+}
+
+TEST_F(MediaRouterEnabledTest, TestDisabledByPolicy) {
+  disabled_profile.GetTestingPrefService()->SetManagedPref(
+      ::prefs::kEnableMediaRouter, std::make_unique<base::Value>(false));
+  EXPECT_FALSE(MediaRouterEnabled(&disabled_profile));
+
+  disabled_profile.GetTestingPrefService()->SetManagedPref(
+      ::prefs::kEnableMediaRouter, std::make_unique<base::Value>(true));
+  // Runtime changes are not supported.
+  EXPECT_FALSE(MediaRouterEnabled(&disabled_profile));
+}
+#endif  // defined(OS_ANDROID) || BUILDFLAG(ENABLE_EXTENSIONS)
+
 }  // namespace media_router