[CrOS PhoneHub] Add comment to remove TriggerRefresh() in the future.

Currently, calling syncer::SyncService::TriggerRefresh() in BrowserTabsModelProvider doesn't guarantee an immediate update.
It bypasses some of the potential sources of latency (e.g. for
delivering an invalidation), but not others (e.g. backend replication
delay). This method of refreshing browser tabs should be replaced when
there is a better means.

Fixed: 1158480
Change-Id: Id3043a3197cfc8dc88f18fb50c8a3e64ccc09a45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618064Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Regan Hsu <hsuregan@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841536}

chrome/browser/chromeos/phonehub/browser_tabs_model_provider_impl.cc

@@ -55,6 +55,12 @@ void BrowserTabsModelProviderImpl::OnHostStatusChanged(
 }
 
 void BrowserTabsModelProviderImpl::TriggerRefresh() {
+  // crbug/1158480: Currently (January 2021), updates to synced sessions
+  // sometimes take a long time to arrive. As a workaround,
+  // SyncService::TriggerRefresh() is used, which bypasses some of the potential
+  // sources of latency (e.g. for delivering an invalidation), but not others
+  // (e.g. backend replication delay). I.e SyncService::TriggerRefresh() will
+  // not guarantee an immediate update.
   sync_service_->TriggerRefresh({syncer::SESSIONS});
 }