The default value of ChromeClientWx::scaleFactor must be 1.0. Otherwise, canvas element won't be painted.
Created attachment 68793 [details] Patch
Comment on attachment 68793 [details] Patch r=me
Comment on attachment 68793 [details] Patch Clearing flags on attachment: 68793 Committed r68319: <http://trac.webkit.org/changeset/68319>
All reviewed patches have been landed. Closing bug.