Working on  crbug.com/840372 , when the encoding quality is set to anything less than 1, like 0.99, Jpeg and Webp encoding seem to end up encoding 0,0,0,a instead of r,g,b,a. This still requires more investigation. Png encoding seems to work fine.

After this is resolved, we need to extend convertToBlob() layout tests with a few <1 quality values.