Block sync commits if updates pending encryption

ModelTypeWorker::GetContribution() should also be blocked if some
updates are pending a decryption key, otherwise a DCHECK is hit:
[32730:356:1120/142053.301971:FATAL:model_type_worker.cc(471)] Check failed: entries_pending_decryption_.empty().

Since this codepath was the only remaining user of BlockForEncryption()
that didn't verify entries_pending_decryption_ being empty, we change
BlockForEncryption() itself to also require that condition.

Bug: 873902,873494
Change-Id: Ic9f841380da5f81dd440d39698902a6fe2966be4
Reviewed-on: https://chromium-review.googlesource.com/c/1346392
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610095}

components/sync/engine_impl/model_type_worker.cc

@@ -392,9 +392,11 @@ void ModelTypeWorker::EncryptionAcceptedMaybeApplyUpdates() {
 }
 
 void ModelTypeWorker::ApplyPendingUpdates() {
-  if (!entries_pending_decryption_.empty())
+  if (BlockForEncryption())
     return;
 
+  DCHECK(entries_pending_decryption_.empty());
+
   DVLOG(1) << ModelTypeToString(type_) << ": "
            << base::StringPrintf("Delivering %" PRIuS " applicable updates.",
                                  pending_updates_.size());
@@ -538,6 +540,9 @@ bool ModelTypeWorker::CanCommitItems() const {
 }
 
 bool ModelTypeWorker::BlockForEncryption() const {
+  if (!entries_pending_decryption_.empty())
+    return true;
+
   // Should be using encryption, but we do not have the keys.
   return cryptographer_ && !cryptographer_->is_ready();
 }