Fix jingle message parser to handle messages from legacy hosts properly.
Fix jingle message parser to handle messages from legacy hosts properly.

Previously JingleMessage::Parse() was rejecting any message that contains
legacy GICE transport info as invalid. That prevented newer clients from
showing correct error message when trying to connect to old GICE-only
hosts, because they always add an empty GICE <transport> tag in
session-accept message. With this fix the client will return correct
error code when connecting to a legacy host.

Review URL: https://codereview.chromium.org/1264713003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#341081}

CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend