Make CreateTrialsFromSeed visible in traces.

VariationsFieldTrialCreator::CreateTrialsFromSeed() function takes
~100ms on Android Go devices (see the bug).

This CL adds TRACE_EVENT to the function to make it visible in startup
traces.

Bug: 761684
Change-Id: I339de72e6e04cfa2ba30e65ce8d5888d467fead2
Reviewed-on: https://chromium-review.googlesource.com/825647Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523964}

components/variations/service/variations_field_trial_creator.cc

@@ -16,6 +16,7 @@
 #include "base/metrics/histogram_macros.h"
 #include "base/metrics/sparse_histogram.h"
 #include "base/sys_info.h"
+#include "base/trace_event/trace_event.h"
 #include "base/version.h"
 #include "build/build_config.h"
 #include "components/prefs/pref_service.h"
@@ -145,6 +146,7 @@ bool VariationsFieldTrialCreator::CreateTrialsFromSeed(
     std::unique_ptr<const base::FieldTrial::EntropyProvider>
         low_entropy_provider,
     base::FeatureList* feature_list) {
+  TRACE_EVENT0("startup", "VariationsFieldTrialCreator::CreateTrialsFromSeed");
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
   CHECK(!create_trials_from_seed_called_);