[Sync] Log dropped invalidations in Sync browser tests

This patch adds logging for the dropped invalidations in Sync browser
tests. This will help investigate some flaky tests.

Invalidation are dropped if they are received before an
FCMNetworkHandler is actually assigned and listening for FCM messages.


Change-Id: I10409bfdda7b0616da6e45a6ecd5f00b3cd5bef0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1771904
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691090}

chrome/browser/sync/test/integration/fake_server_invalidation_sender.cc

@@ -39,6 +39,10 @@ void FakeServerInvalidationSender::OnCommit(
   // FCM network handlers get assigned in SetupInvalidations() which happens
   // after SetupSync().
   if (fcm_network_handler == nullptr) {
+    DLOG(WARNING) << "Received invalidations for the following data types in "
+                     "invalidation sender "
+                  << this << " will be dropped:"
+                  << ModelTypeSetToString(committed_model_types);
     return;
   }
   // For each of the committed model types, pass a message to the FCM Network