I thought PDF.js was the first implementation. Firefox added it in 2012.
Chrome’s PDF viewer launched in Dec 2010[1] with v8.0.552.
The release notes do note that a benefit of the built-in viewer is that it’s sandboxed. That’s fair enough, I guess — the Chrome team had a bee in their bonnet about plugins going all the way back to the Chrome launch comic[2].
Interestingly the Chrome team were using svn back then — that’s how long it’s been :-)
[1] https://chromereleases.googleblog.com/2010/12/stable-beta-ch...
People would even put in “warning: pdf” text when linking to PDFs because of how Adobe Reader slowed people down.