[Extensions] Add a Manifest V3 base::Feature

Add a base::Feature to control Manifest V3 support. If disabled, all
Manifest V3-based extensions will fail to load. This should only be
used as a last resort, and is very much a kill-switch (rather than a
gradual ramp-up tool).

By default, this feature is enabled (allowing manifest v3 extensions).

Bug: 1147344
Change-Id: I3e30ad76fce12bae584a625c81e3759b31d9478e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2520333Reviewed-by: Kelvin Jiang <kelvinjiang@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826985}

extensions/common/extension.cc

@@ -12,6 +12,7 @@
 
 #include "base/base64.h"
 #include "base/command_line.h"
+#include "base/feature_list.h"
 #include "base/files/file_path.h"
 #include "base/i18n/rtl.h"
 #include "base/json/json_writer.h"
@@ -32,6 +33,7 @@
 #include "content/public/common/url_constants.h"
 #include "extensions/common/constants.h"
 #include "extensions/common/error_utils.h"
+#include "extensions/common/extension_features.h"
 #include "extensions/common/feature_switch.h"
 #include "extensions/common/manifest.h"
 #include "extensions/common/manifest_constants.h"
@@ -85,6 +87,15 @@ bool IsManifestSupported(int manifest_version,
   static constexpr int kMaximumSupportedManifestVersion = 2;
   static_assert(kMaximumSupportedManifestVersion >= kModernManifestVersion,
                 "The modern manifest version must be supported.");
+
+  // The ultimate short-circuit: If the feature for MV3 is disabled, it's not
+  // supported.
+  if (manifest_version == 3 &&
+      !base::FeatureList::IsEnabled(
+          extensions_features::kMv3ExtensionsSupported)) {
+    return false;
+  }
+
   // Modern is always safe.
   if (manifest_version >= kModernManifestVersion &&
       manifest_version <= kMaximumSupportedManifestVersion) {

extensions/common/extension_features.cc

@@ -48,6 +48,10 @@ const base::Feature kAllowWithholdingExtensionPermissionsOnInstall{
 const base::Feature kContentScriptsMatchOriginAsFallback{
     "ContentScriptsMatchOriginAsFallback", base::FEATURE_DISABLED_BY_DEFAULT};
 
+// Whether Manifest Version 3-based extensions are supported.
+const base::Feature kMv3ExtensionsSupported{"Mv3ExtensionsSupported",
+                                            base::FEATURE_ENABLED_BY_DEFAULT};
+
 // Reports Extensions.WebRequest.KeepaliveRequestFinished when enabled.
 const base::Feature kReportKeepaliveUkm{"ReportKeepaliveUkm",
                                         base::FEATURE_ENABLED_BY_DEFAULT};

extensions/common/extension_features.h

@@ -27,6 +27,8 @@ extern const base::Feature kAllowWithholdingExtensionPermissionsOnInstall;
 
 extern const base::Feature kContentScriptsMatchOriginAsFallback;
 
+extern const base::Feature kMv3ExtensionsSupported;
+
 extern const base::Feature kReportKeepaliveUkm;
 
 extern const base::Feature kReturnScopesInGetAuthToken;

extensions/common/extension_unittest.cc

@@ -7,6 +7,8 @@
 #include "base/command_line.h"
 #include "base/optional.h"
 #include "base/test/scoped_command_line.h"
+#include "base/test/scoped_feature_list.h"
+#include "extensions/common/extension_features.h"
 #include "extensions/common/manifest_constants.h"
 #include "extensions/common/switches.h"
 #include "extensions/common/value_builder.h"
@@ -126,6 +128,15 @@ TEST(ExtensionTest, ExtensionManifestVersions) {
     EXPECT_TRUE(
         RunManifestVersionSuccess(get_manifest(base::nullopt), kType, 1));
   }
+
+  {
+    // If the requisite feature is disabled, Manifest V3 extensions should
+    // fail to load.
+    base::test::ScopedFeatureList feature_list;
+    feature_list.InitAndDisableFeature(
+        extensions_features::kMv3ExtensionsSupported);
+    EXPECT_TRUE(RunManifestVersionFailure(get_manifest(3)));
+  }
 }
 
 TEST(ExtensionTest, PlatformAppManifestVersions) {