assert in Document::external()

[GoogleCodeExporter]

Seen under run_javascript_tests.py on win: 
http://build.chromium.org/p/client.pdfium/builders/win/builds/7/steps/javascript
%20tests/logs/stdio

Need to check for IsGetting() before using the << operator to return a result.

Original issue reported on code.google.com by tsepez@chromium.org on 27 Mar 2015 at 4:04