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

ios/chrome/browser/ios_chrome_main_parts.mm

Index: ios/chrome/browser/ios_chrome_main_parts.mm
diff --git a/ios/chrome/browser/ios_chrome_main_parts.mm b/ios/chrome/browser/ios_chrome_main_parts.mm
index fab26e1310436dbde7dacd3bfdfaf9a2e75f6174..c510e90b0e8cf7cb23152f1dac3452d9d1443897 100644
--- a/ios/chrome/browser/ios_chrome_main_parts.mm
+++ b/ios/chrome/browser/ios_chrome_main_parts.mm
@@ -9,6 +9,7 @@
 #include "base/files/file_path.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
+#include "base/metrics/user_metrics.h"
 #include "base/path_service.h"
 #include "base/time/default_tick_clock.h"
 #include "components/content_settings/core/browser/cookie_settings.h"
@@ -189,6 +190,10 @@ void IOSChromeMainParts::PostDestroyThreads() {
 
 // This will be called after the command-line has been mutated by about:flags
 void IOSChromeMainParts::SetUpMetricsAndFieldTrials() {
+  // Set the thread on which to execute RecordAction.

[Ilya Sherman, 2016/04/15 22:16:05]

Ditto.

[beaudoin, 2016/04/18 14:29:52]

Done.

+  base::SetRecordActionTaskRunner(
+      web::WebThread::GetTaskRunnerForThread(web::WebThread::UI););
+
   // Must initialize metrics after labs have been converted into switches,
   // but before field trials are set up (so that client ID is available for
   // one-time randomized field trials).