SyncPrefs: Run SyncSuppressed->SyncRequested pref migration also on iOS

Previously, https://crrev.com/c/1681768 added migration logic from the
old "SyncSuppressed" pref to a new "SyncRequested" pref (which better
matches what it means, and has the correct default value). However, the
migration code was only triggered on non-iOS platforms, leaving iOS Sync
users in a bad state (new pref is used, but has no value).
The fix is simply to trigger the migration logic also in iOS's version
of browser_prefs. This also bring users back into a good state.

Bug: 906034
Change-Id: Ib52ea658bf616ca47e947980d2ddc1fb50f02b5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715285Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Marc Treib <treib@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680368}

ios/chrome/browser/prefs/browser_prefs.mm

@@ -195,4 +195,7 @@ void MigrateObsoleteBrowserStatePrefs(PrefService* prefs) {
 
   // Added 10/2018.
   prefs->ClearPref(kReverseAutologinEnabled);
+
+  // Added 07/2019.
+  syncer::MigrateSyncSuppressedPref(prefs);
 }