It is not safe to call GraphicsContext::paintFrameForMedia() off the main thread in the GPUProcess. It relies on the MediaPlayer which is a main-thread object.
Created attachment 429114 [details] Patch
Committed r277774 (237935@main): <https://commits.webkit.org/237935@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 429114 [details].
<rdar://problem/78241935>