The current call in encodeNativeImage() passes an empty FloatSize for the argument 'imageSize'. This causes the NativeImage not be be drawn to the ShareableBitmap. So encoding the NativeImage was always sending zero image from the WebProcess to the GPUProcess.
Created attachment 393277 [details] Patch
This patch lowers the failures of the command run-webkit-tests --debug --no-retry LayoutTests/canvas/ LayoutTests/fast/canvas --internal-feature RenderCanvasInGPUProcessEnabled from 69 to 45 (after the patch of https://bugs.webkit.org/show_bug.cgi?id=208876 and https://bugs.webkit.org/show_bug.cgi?id=208931).
Comment on attachment 393277 [details] Patch Clearing flags on attachment: 393277 Committed r258312: <https://trac.webkit.org/changeset/258312>
All reviewed patches have been landed. Closing bug.
<rdar://problem/60355796>