Bug 1044224 - [Messages][Tests] Generate assets on the fly instead of loading it with XHR. Generating assets using canvas. r=julien

yeah :)
`Promise.all(blobPromises).then(() => done(), done);`

Or I used to do this too:
`Promise.all(blobPromises).then(done.bind(null, null), done);`

(but I prefer the first proposal)