Avoid WrappedSkImageBacking crash

When SharedImageFactory is created for WebGL fallback (aka SwiftShader)
there is no skia context. Don't create WrappedSkImageBackingFactory or
RawDrawImageBackingFactory since they both require a skia context to be
used. There are CHECK failures occurring due to this.

Bug: 325850215
Change-Id: Iae5ed3816841ea289661ce2d3e3bc6a180b3fba4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5527393
Commit-Queue: Kyle Charbonneau <kylechar@chromium.org>
Reviewed-by: Saifuddin Hitawala <hitawala@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1298268}