We previously refactor canvas shutdown to account for the fact that they
needed to be shutdown in conjunction with the DOM worker reference
kept alive by the CanvasManagerChild. Unfortunately if the compositor
process crashes, or otherwise the CanvasManagerChild actor is torn down,
we also prematurely shutdown the canvas when it would previously
fallback to Skia in the content process.
This patch abstracts out canvas shutdown into the CanvasShutdownManager
which has the owning reference to the ThreadSafeWorkerRef. It corrects a
similar bug on the main thread as well for HTMLCanvasElement.
Differential Revision: https://phabricator.services.mozilla.com/D204988
0a149a309c