AW UMA: guard stability metrics behind a flag

This guards stability metrics behind kWebViewWakeMetricsService. While
not directly related, AwStabilityMetricsProvider has the same effect as
enabling this flag because it logs a "user action" for every page load.

This puts the entire metrics provider behind the off-by-default flag, to
avoid any accidental UMA volume increase until we've found a path
forward for launching that feature.

Bug: 1058684
Test: None
Change-Id: I0937a1d44aaf4d0c54dee8dfe4644c2216d77891
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095196
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Tobias Sargeant <tobiasjs@chromium.org>
Reviewed-by: Tobias Sargeant <tobiasjs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748333}

android_webview/browser/metrics/aw_metrics_service_client.cc

@@ -175,9 +175,11 @@ void AwMetricsServiceClient::OnAppStateChanged(
 
 void AwMetricsServiceClient::RegisterAdditionalMetricsProviders(
     metrics::MetricsService* service) {
-  service->RegisterMetricsProvider(
-      std::make_unique<android_webview::AwStabilityMetricsProvider>(
-          pref_service()));
+  if (base::FeatureList::IsEnabled(features::kWebViewWakeMetricsService)) {
+    service->RegisterMetricsProvider(
+        std::make_unique<android_webview::AwStabilityMetricsProvider>(
+            pref_service()));
+  }
 }
 
 // static