[Not for review] Add TRACE_EVENTs and chrome://flags for preconnect

chrome/browser/net/predictor.cc

Index: chrome/browser/net/predictor.cc
diff --git a/chrome/browser/net/predictor.cc b/chrome/browser/net/predictor.cc
index 5d73b277c1d45cc6ef19e618cbbd8d52a8f2c0b9..7b020d6a3c1de5f35fc2d9424195ddf7f9a8f6b9 100644
--- a/chrome/browser/net/predictor.cc
+++ b/chrome/browser/net/predictor.cc
@@ -27,6 +27,7 @@
 #include "base/threading/thread_restrictions.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/time/time.h"
+#include "base/trace_event/trace_event.h"
 #include "base/values.h"
 #include "chrome/browser/io_thread.h"
 #include "chrome/browser/prefs/session_startup_pref.h"
@@ -760,7 +761,8 @@ void Predictor::PreconnectUrlOnIOThread(
 
   if (!profile_io_data_)
     return;
-
+  TRACE_EVENT2("net", "Predictor::PreconnectUrlOnIOThread", "url",
+               original_url.spec(), "motivation", static_cast<int>(motivation));
   // Translate the motivation from UrlRequest motivations to HttpRequest
   // motivations.
   net::HttpRequestInfo::RequestMotivation request_motivation =
@@ -839,6 +841,8 @@ enum SubresourceValue {
 
 void Predictor::PrepareFrameSubresources(const GURL& original_url,
                                          const GURL& first_party_for_cookies) {
+  TRACE_EVENT1("net", "Predictor::PrepareFrameSubresources", "url",
+               original_url.spec());
   // Apply HSTS redirect early so it is taken into account when looking up
   // subresources.
   GURL url = GetHSTSRedirectOnIOThread(original_url);