Remove kMediaEngagementSchemaVersion from incognito persistent list.

kMediaEngagementSchemaVersion is removed from the list of preferences
that are persieted if modified in incognito mode.

Bug: 861722
Change-Id: Idb74a3d2013d74c5c7158a10aca1fb88b926c6db
Reviewed-on: https://chromium-review.googlesource.com/1237974Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593481}

chrome/browser/prefs/pref_service_incognito_whitelist.cc

@@ -257,8 +257,6 @@ const char* const kTemporaryIncognitoWhitelist[] = {
 #endif
 
     prefs::kBackgroundTracingLastUpload,
-
-    prefs::kMediaEngagementSchemaVersion,
 };
 
 }  // namespace