Chromecast: rm explicit constructor on CastMetricsServiceClient.

R=lcwu@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/521943002

Cr-Commit-Position: refs/heads/master@{#292736}

chromecast/metrics/cast_metrics_service_client.h

@@ -54,7 +54,7 @@ class CastMetricsServiceClient : public ::metrics::MetricsServiceClient {
   void EnableMetricsService(bool enabled);
 
  private:
-  explicit CastMetricsServiceClient(
+  CastMetricsServiceClient(
       PrefService* pref_service,
       net::URLRequestContextGetter* request_context);