Code:   void OnReadAllMetadata(
      const SessionStore::SessionInfo& session_info,
      SessionStore::FactoryCompletionCallback callback,
      std::unique_ptr<ModelTypeStore> store,
       std::unique_ptr<ModelTypeStore::RecordList> record_list,
       const base::Optional<syncer::ModelError>& error,
       std::unique_ptr<syncer::MetadataBatch> metadata_batch) {
     if (error) {
       std::move(callback).Run(error, /*store=*/nullptr,
                               /*metadata_batch=*/nullptr);
      return;
    }

    std::map<std::string, sync_pb::SessionSpecifics> initial_data;
    for (ModelTypeStore::Record& record : *record_list) {
      const std::string& storage_key = record.id;
      SessionSpecifics specifics;
      if (storage_key.empty() ||
          !specifics.ParseFromString(std::move(record.value))) {
        DVLOG(1) << "Ignoring corrupt database entry with key: " << storage_key;
        continue;
      }
      initial_data[storage_key].Swap(&specifics);
    }

    auto session_store = std::make_unique<SessionStore>(
        sessions_client_, session_info, std::move(store),
        std::move(initial_data), metadata_batch->GetAllMetadata(),
        restored_foreign_tab_callback_);

    std::move(callback).Run(/*error=*/base::nullopt, std::move(session_store),
                            std::move(metadata_batch));
  }

Vulnerability Type: 
CWE ID: CWE-20
Summary: Incorrect handling of alert box display in Blink in Google Chrome prior to 71.0.3578.80 allowed a remote attacker to present confusing browser UI via a crafted HTML page.
Commit Message: Add trace event to sync_sessions::OnReadAllMetadata()

It is likely a cause of janks on UI thread on Android.
Add a trace event to get metrics about the duration.

BUG=902203

Change-Id: I4c4e9c2a20790264b982007ea7ee88ddfa7b972c
Reviewed-on: https://chromium-review.googlesource.com/c/1319369
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: ssid <ssid@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606104}