Move the thread hop for UMA user actions from content:: to base::.

android_webview/browser/aw_metrics_service_client.h

Index: android_webview/browser/aw_metrics_service_client.h
diff --git a/android_webview/browser/aw_metrics_service_client.h b/android_webview/browser/aw_metrics_service_client.h
index 29cca691e4b86dfd03acb85b8901e6e1e4257337..d6021b0738029e912ffdd6e733b420b53b8e0063 100644
--- a/android_webview/browser/aw_metrics_service_client.h
+++ b/android_webview/browser/aw_metrics_service_client.h
@@ -66,6 +66,8 @@ class AwMetricsServiceClient : public metrics::MetricsServiceClient {
   std::unique_ptr<metrics::MetricsLogUploader> CreateUploader(
       const base::Callback<void(int)>& on_upload_complete) override;
   base::TimeDelta GetStandardUploadInterval() override;
+  scoped_refptr<base::SingleThreadTaskRunner>
+      GetMainThreadTaskRunner() override;
 
  private:
   AwMetricsServiceClient();