Third resizable header causes TypeError

[pekam]

Description

When grid has more than two layers of headers on top of each other, trying to resize columns by dragging from the third or upper layer header throws:

Uncaught TypeError: Cannot read property 'offsetWidth' of undefined
    at HTMLElement._onHeaderTrack (vaadin-grid-combined-mixin.html:407)
    at Object._fire (gestures.html:696)
    at Object._fire (gestures.html:1038)
    at movefn (gestures.html:935)
    at HTMLDocument.upfn (gestures.html:943)

Expected outcome

Can drag to resize on any header

Actual outcome

Dragging on third or upper header causes TypeError and dragging has no effect

Live Demo

https://glitch.com/edit/#!/friendly-hydrogen

Steps to reproduce

1. Add column-group on top of a column-group on top of a column
2. Make all columns and column-groups resizable
3. Try to resize the top-most headers by dragging

Browsers Affected

• Chrome
• Firefox
• Safari
• Edge
• IE 11
• iOS Safari
• Android Chrome

[web-padawan]

Need to verify if it happens on master also.