[Blink] Implement IsResourceProviderValid() in BaseRC2D

This method is currently not implemented by OffscreenCanvasRC2D,
meaning it using the default NOTREACHED() impl. However, it is called by
BaseRC2D [1]. [1] is currently not called within the context of
OffscreenCanvas, but there's no inherent reason for that. This CL fixes
the fragility here.

Note: CanvasRenderingContext2D::GetResourceProvider() returns null if
canvas() is null [2]. This is not relevant here, as
PaintRenderingResultsToSnapshot() is only called by
HTMLCanvasElement/OffscreenCanvas.

[1] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/modules/canvas/canvas2d/base_rendering_context_2d.cc;l=810-812;drc=0177114415e98506c6628baeee472cd2f0be6c34;bpv=1;bpt=1
[2] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/modules/canvas/canvas2d/canvas_rendering_context_2d.cc;l=1316;drc=0177114415e98506c6628baeee472cd2f0be6c34

Bug: 352263194
Change-Id: I849ec8d15ab3a6295418bf24ab19c0d1adb0346c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7008327
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1525488}