Transmit a X-Chrome-UMA-Enabled bit to Google domains from clients that have UMA enabled.

chrome/browser/profiles/profile_impl_io_data.cc

Index: chrome/browser/profiles/profile_impl_io_data.cc
diff --git a/chrome/browser/profiles/profile_impl_io_data.cc b/chrome/browser/profiles/profile_impl_io_data.cc
index 3345c85ef8d10bee3499396275ef6285162a0e9e..3daa39cabdb37c79a59c23e7ddde07c86ee49254 100644
--- a/chrome/browser/profiles/profile_impl_io_data.cc
+++ b/chrome/browser/profiles/profile_impl_io_data.cc
@@ -113,6 +113,10 @@ void ProfileImplIOData::Handle::Init(
                                              local_state,
                                              io_thread,
                                              main_request_context_getter_);
+
+  io_data_->enable_metrics()->Init(prefs::kMetricsReportingEnabled,
+                                   local_state, NULL);
+  io_data_->enable_metrics()->MoveToThread(BrowserThread::IO);
 }
 
 base::Callback<ChromeURLDataManagerBackend*(void)>