Part of the Windows SDK.
I saw no evidence in the project showing that this file has BSD-style license.And since it is part of the windows SDK, it is nearly impossible to be BSD-style licensed.
Maybe it was included from foxit's code or other codebase but it is better to be put into the thirdparties directory due to the license issue.
Edited:
Thanks for pointing out my misconception about Windows SDK.
Agreed it is always nice to have these things in a thirdparty directory, though, but the larger Chromium project actually does appear to have all of pdfium in third_party, which helps keep that clear.