[STTS] fix crahing bug with MRU erasure

Bug: 1054551
Change-Id: Ied951dec94c6d1d38610deaf61d3842741f38e7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2067191
Auto-Submit: Jeffrey Cohen <jeffreycohen@chromium.org>
Reviewed-by: sebsg <sebsg@chromium.org>
Commit-Queue: sebsg <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744273}

components/send_tab_to_self/send_tab_to_self_bridge.cc

@@ -208,6 +208,9 @@ base::Optional<syncer::ModelError> SendTabToSelfBridge::ApplySyncChanges(
     if (change->type() == syncer::EntityChange::ACTION_DELETE) {
       LogApplySyncChangesStatus(UMAApplySyncChangesStatus::DELETE);
       if (entries_.find(guid) != entries_.end()) {
+        if (mru_entry_ && mru_entry_->GetGUID() == guid) {
+          mru_entry_ = nullptr;
+        }
         entries_.erase(change->storage_key());
         batch->DeleteData(guid);
         removed.push_back(change->storage_key());