Fix NOTREACHED for ProduceSkia()

ProduceSkia() should never be called when GrContextType is kNone, aka
when GPU accelerated compositing is disabled and skia isn't initialized
in the GPU process. Renderer/extension processes are notified about GPU
accelerated compositing being disabled asynchronously so they could
issue raster work that tries to ProduceSkia() before that point and hit
the not reached. Change to just return null in this case to signal
failure without crashing.

Fixed: 356512109
Change-Id: Ia74149f2136b93b6c07eb5f9de80c45b22ce53ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5753784
Reviewed-by: Peter McNeeley <petermcneeley@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Kyle Charbonneau <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1335686}