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

chrome/browser/chrome_browser_field_trials.cc

Index: chrome/browser/chrome_browser_field_trials.cc
diff --git a/chrome/browser/chrome_browser_field_trials.cc b/chrome/browser/chrome_browser_field_trials.cc
index 313bff3a56112f0ba747d091a88776437cc640e8..69fac07e38c02d44ee3bab2be872a35d54c4a30a 100644
--- a/chrome/browser/chrome_browser_field_trials.cc
+++ b/chrome/browser/chrome_browser_field_trials.cc
@@ -73,9 +73,7 @@
   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