Enable ExternalExperimentAllowlist feature by default.

This enables the allowlist feature implemented here:
https://chromium-review.googlesource.com/c/chromium/src/+/2324172

Bug: 1111941
Change-Id: Id0110e18b277e2750f9b0a7a2e995ec6be0649f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2341893Reviewed-by: Peter Conn <peconn@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795892}

components/variations/synthetic_trial_registry.cc

@@ -16,7 +16,7 @@ namespace variations {
 namespace internal {
 
 const base::Feature kExternalExperimentAllowlist{
-    "ExternalExperimentAllowlist", base::FEATURE_DISABLED_BY_DEFAULT};
+    "ExternalExperimentAllowlist", base::FEATURE_ENABLED_BY_DEFAULT};
 
 }  // namespace internal