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}
bool SessionStore::StorageKeyMatchesLocalSession(
    const std::string& storage_key) const {
  std::string session_tag;
  int tab_node_id;
  bool success = DecodeStorageKey(storage_key, &session_tag, &tab_node_id);
  DCHECK(success);
  return session_tag == local_session_info_.session_tag;
}

bool SessionStore::StorageKeyMatchesLocalSession(
    const std::string& storage_key) const {
  std::string session_tag;
  int tab_node_id;
  bool success = DecodeStorageKey(storage_key, &session_tag, &tab_node_id);
  DCHECK(success);
  return session_tag == local_session_info_.session_tag;
}
