Set allow-CRX2 to default to false again.

This appears to have been accidentally reverted to "true" in a bad merge.

Bug: 740715
Change-Id: I6219ab58555b44b258b9e1af32d010ada33a34d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574945Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Joshua Pawlicki <waffles@chromium.org>
Auto-Submit: Joshua Pawlicki <waffles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652571}

extensions/browser/extension_prefs.cc

@@ -1869,7 +1869,7 @@ void ExtensionPrefs::RegisterProfilePrefs(
                                 true);
   registry->RegisterIntegerPref(kCorruptedDisableCount, 0);
   registry->RegisterBooleanPref(pref_names::kInsecureExtensionUpdatesEnabled,
-                                true);
+                                false);
   registry->RegisterBooleanPref(pref_names::kUninstallBlacklistedExtensions,
                                 false);