[NOT FOR REVIEW] Instrument net/ with trace events.

net/dns/host_resolver_impl.cc

Index: net/dns/host_resolver_impl.cc
diff --git a/net/dns/host_resolver_impl.cc b/net/dns/host_resolver_impl.cc
index bc1617c227e73598daa63a71372020f459e9d0c1..d64bcc7eb7a7756fa9a44add2d0304a746e37105 100644
--- a/net/dns/host_resolver_impl.cc
+++ b/net/dns/host_resolver_impl.cc
@@ -35,6 +35,7 @@
 #include "base/thread_task_runner_handle.h"
 #include "base/threading/worker_pool.h"
 #include "base/time/time.h"
+#include "base/trace_event/trace_event.h"
 #include "base/values.h"
 #include "net/base/address_family.h"
 #include "net/base/address_list.h"
@@ -764,6 +765,7 @@ class HostResolverImpl::ProcTask
                         const uint32_t attempt_number,
                         int error,
                         const int os_error) {
+    TRACE_EVENT0("net", "net::ProcTask::OnLookupComplete");
     DCHECK(task_runner_->BelongsToCurrentThread());
     // If results are empty, we should return an error.
     bool empty_list_on_ok = (error == OK && results.empty());