Use TimeTicks as much as possible in startup_metric_utils.

chrome/browser/mac/mac_startup_profiler.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/mac/mac_startup_profiler.h"
 
 #include "base/logging.h"
 #include "base/metrics/histogram.h"
 #include "components/startup_metric_utils/browser/startup_metric_utils.h"
 
 // static
 MacStartupProfiler* MacStartupProfiler::GetInstance() {
   return base::Singleton<MacStartupProfiler>::get();
 }
 
 MacStartupProfiler::MacStartupProfiler() : recorded_metrics_(false) {
 }
 
 MacStartupProfiler::~MacStartupProfiler() {
 }
 
 void MacStartupProfiler::Profile(Location location) {
-  profiled_times_[location] = base::Time::Now();
+  profiled_ticks_[location] = base::TimeTicks::Now();
 }
 
 void MacStartupProfiler::RecordMetrics() {
-  const base::Time main_entry_time = startup_metric_utils::MainEntryPointTime();
-  DCHECK(!main_entry_time.is_null());
+  const base::TimeTicks main_entry_ticks =
+      startup_metric_utils::MainEntryPointTicks();
+  DCHECK(!main_entry_ticks.is_null());
   DCHECK(!recorded_metrics_);
 
   recorded_metrics_ = true;
 
-  for (std::map<Location, base::Time>::const_iterator it =
-           profiled_times_.begin();
-       it != profiled_times_.end();
-       ++it) {
-    const base::Time& location_time = it->second;
-    base::TimeDelta delta = location_time - main_entry_time;
-    RecordHistogram(it->first, delta);
+  for (const auto& entry : profiled_ticks_) {
+    const Location location = entry.first;

[msw, 2015/11/10 18:58:37]

nit: could this also be a const ref? (or inline [both and remove curlies]?)

[gab, 2015/11/10 19:18:49]

It's an enum, refs for enums don't make much sense.

I don't think the types add clarity though I guess that makes a case for not
using auto...

Changed for explicit type, with one line for-each loop.

+    const base::TimeTicks& location_ticks = entry.second;
+    RecordHistogram(location, location_ticks - main_entry_ticks);
   }
 }
 
 const std::string MacStartupProfiler::HistogramName(Location location) {
   std::string prefix("Startup.OSX.");
   switch (location) {
     case PRE_MAIN_MESSAGE_LOOP_START:
       return prefix + "PreMainMessageLoopStart";
     case AWAKE_FROM_NIB:
       return prefix + "AwakeFromNib";
@@ skipping 21 matching lines @@
   // method will be the first and only usage of a histogram with that given
   // name.
   base::HistogramBase* histogram = base::Histogram::FactoryTimeGet(
       name,
       min,
       max,
       bucket_count,
       base::HistogramBase::kUmaTargetedHistogramFlag);
   histogram->AddTime(delta);
 }