Modify instrumentation for various jank bugs, mostly removing outdated sites.

net/url_request/url_request.cc

Index: net/url_request/url_request.cc
diff --git a/net/url_request/url_request.cc b/net/url_request/url_request.cc
index 4a1e09346d8e936b577b60eeda2e77f42b838b50..5786117063e188110a86bdd529f6de0656516ae5 100644
--- a/net/url_request/url_request.cc
+++ b/net/url_request/url_request.cc
@@ -487,6 +487,10 @@ void URLRequest::set_delegate(Delegate* delegate) {
 }
 
 void URLRequest::Start() {
+  // TODO(pkasting): Remove ScopedTracker below once crbug.com/456327 is fixed.
+  tracked_objects::ScopedTracker tracking_profile(
+      FROM_HERE_WITH_EXPLICIT_FUNCTION("456327 URLRequest::Start"));
+
   // Some values can be NULL, but the job factory must not be.
   DCHECK(context_->job_factory());