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

chrome/browser/metrics/chrome_metrics_service_client.cc

Index: chrome/browser/metrics/chrome_metrics_service_client.cc
diff --git a/chrome/browser/metrics/chrome_metrics_service_client.cc b/chrome/browser/metrics/chrome_metrics_service_client.cc
index 19fc1f8133aeb13984c2b0b19c130e2690b742ac..d4b3b0d947244dbe954f7bfcadc4d286d12f300a 100644
--- a/chrome/browser/metrics/chrome_metrics_service_client.cc
+++ b/chrome/browser/metrics/chrome_metrics_service_client.cc
@@ -345,6 +345,12 @@ ChromeMetricsServiceClient::GetDefaultOptIn() {
   return GetMetricsReportingDefaultOptIn(g_browser_process->local_state());
 }
 
+scoped_refptr<base::SingleThreadTaskRunner>
+ChromeMetricsServiceClient::GetMainThreadTaskRunner() {
+  return content::BrowserThread::GetMessageLoopProxyForThread(
+      content::BrowserThread::UI);
+}
+
 void ChromeMetricsServiceClient::Initialize() {
   // Clear metrics reports if it is the first time cellular upload logic should
   // apply to avoid sudden bulk uploads. It needs to be done before initializing