The ShareableBitmapCG aligns the rows using IOSurfaceAlignProperty(kIOSurfaceBytesPerRow, ...). This means the default implementation which is "width x 4" is wrong for ImageBufferShareableBitmapBackend.
This will fix the layout failure:
fast/canvas/canvas-large-dimensions.html [ Failure ]
Created attachment 421390 [details]
Committed r273406: <https://commits.webkit.org/r273406>
All reviewed patches have been landed. Closing bug and clearing flags on attachment 421390 [details].