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
Chromium just automatically embeds a page with PDF, the custom plugin is bundled up with Chrome. Adobe and Google has some mutual agreement:
You can right click and inspect the PDF Native Viewer to see yourself, you will see something like the following:
<html>
<body marginwidth="0" marginheight="0" bgcolor="rgb(38,38,38)">
<embed width="100%" height="100%" name="plugin" src="http://path/to/file.pdf" type="application/pdf">
</body>
</html>