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.
The SDK has a very permissive license (including redistribution), since it's obviously designed to be used, and it's convenient to developers to be able to redistribute parts of it to ease development.
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.
https://pdfium.googlesource.com/pdfium/+/master/core/src/fxg...
"/\ * * Copyright (c) 1998-2000, Microsoft Corp. All Rights Reserved. * * Module Name: * * Gdiplus.h * * Abstract: * * GDI+ Native C++ public header file * \/ "