[TrustedVault] Consume kFollowTrustedVaultKeyRotation

Check of kFollowTrustedVaultKeyRotation was accidentally removed. It
currently doesn't lead to accidental enabling of the feature, because
the feature requires specific command-line setup, but seems dangerous.

Bug: 1113598
Change-Id: I298b73f2e6569232693134e8f0e0812d1121498e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2627154
Commit-Queue: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844066}

components/sync/trusted_vault/standalone_trusted_vault_client.cc

@@ -147,7 +147,8 @@ StandaloneTrustedVaultClient::StandaloneTrustedVaultClient(
   std::unique_ptr<TrustedVaultConnection> connection;
   GURL trusted_vault_service_gurl =
       ExtractTrustedVaultServiceURLFromCommandLine();
-  if (trusted_vault_service_gurl.is_valid()) {
+  if (base::FeatureList::IsEnabled(switches::kFollowTrustedVaultKeyRotation) &&
+      trusted_vault_service_gurl.is_valid()) {
     connection = std::make_unique<TrustedVaultConnectionImpl>(
         trusted_vault_service_gurl, url_loader_factory->Clone(),
         std::make_unique<TrustedVaultAccessTokenFetcherImpl>(