Add Precache.Fetch.Begin user action.

components/precache/content/precache_manager.cc

Index: components/precache/content/precache_manager.cc
diff --git a/components/precache/content/precache_manager.cc b/components/precache/content/precache_manager.cc
index e758a4bbbcc88dc3f75b82ef72c3f5446cca1eac..d9ad0ae9809a7c19d826b8fb0e894b42a83e1df2 100644
--- a/components/precache/content/precache_manager.cc
+++ b/components/precache/content/precache_manager.cc
@@ -14,6 +14,8 @@
 #include "base/memory/ref_counted.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram_macros.h"
+#include "base/metrics/user_metrics.h"
+#include "base/metrics/user_metrics_action.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "base/time/time.h"
@@ -284,6 +286,8 @@ void PrecacheManager::OnGetUnfinishedWorkDone(
   unfinished_work_ = std::move(unfinished_work);
   bool needs_top_hosts = unfinished_work_->top_host_size() == 0;
 
+  base::RecordAction(base::UserMetricsAction("Precache.Fetch.Begin"));
+
   if (IsInExperimentGroup()) {
     BrowserThread::PostTask(
         BrowserThread::DB, FROM_HERE,