I think things might well have changed in 18.04 as in 16.04 it was possible to drag and drop a file from nautilus into Chromium and edit it directly from there as outlined in this thread.
Something has changed between versions (has Chromium itself changed or the fact the google-drive is now automounted using google-drive-occamlfuse).
What I have to do now in 18.04 is open a new tab in Chromium and go to
(as appropriate for the type of document you are trying to open) and provided you have set the document to be available offline, you can click on the document listed and open it and edit it in Chromium.
You may need first to install and enable google docs offline extension for this to work. This extension makes it possible to edit your documents, spreadsheets and presentations when you aren’t connected to the internet.