Fix comment and dead code for handling error in prefs serialization

A previous change added error handling for when prefs serialization
fails.  The check call will result in a crash so I am updating the
comment above it to reflect that and remove the unreachable code
below it.

Change-Id: If16aff75e100c3a92e27344203ccad6d292caf5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1856878
Commit-Queue: Chris Davis <chrdavis@microsoft.com>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705242}

components/prefs/json_pref_store.cc

@@ -482,12 +482,11 @@ bool JsonPrefStore::SerializeData(std::string* output) {
   const bool success = serializer.Serialize(*prefs_);
   if (!success) {
     // Failed to serialize prefs file. Backup the existing prefs file and
-    // reset our existing prefs.
+    // crash.
     BackupPrefsFile(path_);
     CHECK(false) << "Failed to serialize preferences : " << path_
                  << "\nBacked up under "
                  << path_.ReplaceExtension(kBadExtension);
-    prefs_.reset(new base::DictionaryValue());
   }
   return success;
 }