On Sunday and Monday I shared a couple of videos about making narrated Google Earth tours. Unfortunately, Google Earth doesn't work on a Chromebook. So if you want your students to create narrated map tours, the best thing to do is to have them map a series of placemarks in Google's My Maps tool then record a screencast in which they talk about the places on their maps.
You can learn how to use Google's My Maps tools by watching the video embedded below.
A couple of good options for recording a screencast on a Chromebook are Nimbus Screenshot and Capture Cast.