Can't seem to find it in trunk. Is this actually part of the open-source Chromium project? Or is the source to this not actually released? Only mention I could find:
http://www.listware.net/201007/chromium-discuss/93274-chromium-discuss-chrome-pdf-viewer-source.html
Fetch it with Git:
$ git clone https://pdfium.googlesource.com/pdfium
View and contribute: