[chromecast] ExternalMetrics has to destroy itself in file thread.

chromecast/browser/metrics/external_metrics.cc

Index: chromecast/browser/metrics/external_metrics.cc
diff --git a/chromecast/browser/metrics/external_metrics.cc b/chromecast/browser/metrics/external_metrics.cc
index ce9884cb37dba966e1fae70867ac48ae3181d9a0..dae2d2d13fb8887dea3f37ebfbea2a41160b925f 100644
--- a/chromecast/browser/metrics/external_metrics.cc
+++ b/chromecast/browser/metrics/external_metrics.cc
@@ -58,7 +58,13 @@ ExternalMetrics::ExternalMetrics(
   DCHECK(stability_provider);
 }
 
-ExternalMetrics::~ExternalMetrics() {}
+ExternalMetrics::~ExternalMetrics() {
+}
+
+void ExternalMetrics::StopAndSelfDestroy() {
+  content::BrowserThread::DeleteSoon(
+      content::BrowserThread::FILE, FROM_HERE, this);
+}
 
 void ExternalMetrics::Start() {
   ScheduleCollector();