Revert of Make MappedFile (aka OnDisk) the default for persistent histograms.

chrome/browser/chrome_browser_field_trials.cc

 // Copyright (c) 2012 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/chrome_browser_field_trials.h"
 
 #include <string>
 
 #include "base/command_line.h"
 #include "base/feature_list.h"
@@ skipping 55 matching lines @@
   InitResult result;
 
   // Create persistent/shared memory and allow histograms to be stored in
   // it. Memory that is not actualy used won't be physically mapped by the
   // system. BrowserMetrics usage, as reported in UMA, peaked around 3.0MiB
   // as of 2016-12-20.
   const size_t kAllocSize = 5 << 20;     // 5 MiB
   const uint32_t kAllocId = 0x935DDD43;  // SHA1(BrowserMetrics)
   std::string storage = variations::GetVariationParamValueByFeature(
       base::kPersistentHistogramsFeature, "storage");
-
-  // As of M58, "MappedFile" is the default.
-  if (storage.empty() || storage == "MappedFile") {
+  if (storage == "MappedFile") {
     // If for some reason the existing "active" file could not be moved above
     // then it is essential it be scheduled for deletion when possible and the
     // contents ignored. Because this shouldn't happen but can on an OS like
     // Windows where another process reading the file (backup, AV, etc.) can
     // prevent its alteration, it's necessary to handle this case by switching
     // to the equivalent of "LocalMemory" for this run.
     if (base::PathExists(active_file)) {
       base::File file(active_file, base::File::FLAG_OPEN |
                                        base::File::FLAG_READ |
                                        base::File::FLAG_DELETE_ON_CLOSE);
@@ skipping 77 matching lines @@
     bool has_seed,
     base::FeatureList* feature_list) {
   CreateFallbackSamplingTrialIfNeeded(has_seed, feature_list);
 }
 
 void ChromeBrowserFieldTrials::InstantiateDynamicTrials() {
   // Persistent histograms must be enabled as soon as possible.
   InstantiatePersistentHistograms();
   tracing::SetupBackgroundTracingFieldTrial();
 }