Support experiment configurations with/without stability metrics.

components/metrics/file_metrics_provider.cc

 // Copyright 2016 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 "components/metrics/file_metrics_provider.h"
 
 #include "base/command_line.h"
 #include "base/files/file.h"
 #include "base/files/file_enumerator.h"
 #include "base/files/file_util.h"
@@ skipping 88 matching lines @@
   bool read_complete = false;
 
   // Once a file has been recognized as needing to be read, it is mapped
   // into memory and assigned to an |allocator| object.
   std::unique_ptr<base::PersistentHistogramAllocator> allocator;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(SourceInfo);
 };
 
+bool FileMetricsProvider::stability_metrics_enabled_ = true;
+
 FileMetricsProvider::FileMetricsProvider(
     const scoped_refptr<base::TaskRunner>& task_runner,
     PrefService* local_state)
     : task_runner_(task_runner),
       pref_service_(local_state),
       weak_factory_(this) {
 }
 
 FileMetricsProvider::~FileMetricsProvider() {}
 
@@ skipping 344 matching lines @@
   // being reported in RecordInitialHistogramSnapshots because the data
   // will continue to be used by the caller after that method returns. Once
   // here, though, all actions to be done on the data have been completed.
   for (const std::unique_ptr<SourceInfo>& source : sources_for_previous_run_)
     DeleteFileAsync(source->path);
   sources_for_previous_run_.clear();
 }
 
 bool FileMetricsProvider::HasInitialStabilityMetrics() {
   DCHECK(thread_checker_.CalledOnValidThread());
+  if (!stability_metrics_enabled_)

[Alexei Svitkine (slow), 2016/11/25 19:44:42]

Instead of the extra boolean, why not check the param directly here? (You can
use a helper function in the anon namespace to make the call site simple).

This way, it avoids extra complexity of a static and having to set it from
somewhere else.

[bcwhite, 2016/11/28 15:35:46]

I could.  I thought it would be better to keep experiment access in one place. 
There's nothing else in this class that knows about experiments.

[Alexei Svitkine (slow), 2016/11/28 16:08:49]

All other things being equal, I agree.

However, here it's a trade off between making the API surface more complex vs.
having the code to check experiment params be in separate files.

I think it would be better to keep the API surface better at the cost of
checking the params in two different places.

[bcwhite, 2016/11/29 19:09:07]

Done.

+    return false;
 
   // Measure the total time spent checking all sources as well as the time
   // per individual file. This method is called during startup and thus blocks
   // the initial showing of the browser window so it's important to know the
   // total delay.
   SCOPED_UMA_HISTOGRAM_TIMER("UMA.FileMetricsProvider.InitialCheckTime.Total");
 
   // Check all sources for previous run to see if they need to be read.
   for (auto iter = sources_for_previous_run_.begin();
        iter != sources_for_previous_run_.end();) {
@@ skipping 61 matching lines @@
 
     // Dump all histograms contained within the source to the snapshot-manager.
     RecordHistogramSnapshotsFromSource(snapshot_manager, source.get());
 
     // Update the last-seen time so it isn't read again unless it changes.
     RecordSourceAsRead(source.get());
   }
 }
 
 }  // namespace metrics