I believe this design was invented by McCrickard and Catrambone of the Georgia Institute of Technology:
McCrickard DS & Catrambone R (1999). Beyond the scrollbar: An
evolution and evaluation of alternative navigation techniques.
Proceedings of the 1999 IEEE Symposium on Visual Languages, p270-277.
It seems very similar ideas were independently developed in Nordic countries:
Laakso SA, Laakso K, & Saura AJ (2000). Improved scroll bars.
Proceedings of Human Factors in Computing Systems (CHI 00),
Bjork S (2001). The ScrollSearcher Technique: Using Scrollbars to
Explore Search Results. Proceedings of Interact 2001, Eighth IFIP
TC.13 Conference on Human-Computer Interaction.
Bork S & Redstrom J (2002). Window Frames as Areas for Information
Visualization. Proceedings of NordiCHI, p247-250.
In all cases, the authors don’t provide a general term for graphically encoding content information in the scrollbar track, but instead give different names depending on exactly what is encoded. Chrome’s feature, for example, would be called a ScrollSearcher. There are also ChangeIndicator, ReadabilityViewer, Bookmark Scroll, Calendar Scroll, Mural Bar, and Pile Bar.
We could call them “Georgian Bars” in honor of their birthplace, at least until someone finds an earlier example than McCrickard and Catrambone. A simply descriptive name would be “encoded content scrollbars.”