Fix in adapter.js for chromium builds on Mac which don't have version.

Fix in adapter.js for chromium builds on Mac that don't have version
info available. See http://crbug.com/374263.
Fix in adapter.js for chromium builds on Mac which don't have version info.

Updated with Justin's suggestion.
Merge pull request #43 from GoogleChrome/webrtc_adapter_version_fix

Fix in adapter.js for chromium builds on Mac which don't have version info.