I have an FR15 and chromebook.
I just found out that I can connect the FR15 to the chromebook with the USB cable.
Using chromebook "files" app I can see a *.fit file.
Using the Chrome web browser I log onto http://connect.garmin.com/modern/
On the left side panel and under 'activities' the screen pops up and there is an "import" data
Click, surf to the garmin device, and select the .fit file.
It uploads
Get back to us when you get Garmin Express to run on your Chromebook. ...YOU CAN NOT Sync a FR15 with a Chromebook!!!!!