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}
std::string SessionStore::GetStorageKey(const SessionSpecifics& specifics) {
  DCHECK(AreValidSpecifics(specifics));
  return EncodeStorageKey(specifics.session_tag(), specifics.tab_node_id());
}

std::string SessionStore::GetStorageKey(const SessionSpecifics& specifics) {
  DCHECK(AreValidSpecifics(specifics));
  return EncodeStorageKey(specifics.session_tag(), specifics.tab_node_id());
}
