Move CanvasRenderTask to gfx crate.
Update expected results for send-content-type-charset.htm.
Merge pull request #2934 from saneyuki/canvas

Move CanvasRenderTask to gfx crate.
Merge pull request #2991 from Ms2ger/send-content-type-charset

Update expected results for send-content-type-charset.htm; r=jdm