[Blink] Rework CanvasRenderingContext::IsCanva2DResourceValid()

* Rationalize it to just check whether the resource provider exists and
  is valid
* Rename it to IsCanvas2DResourceProviderValid() for clarity
* Use it in HTMLCanvasElement
* Update CanvasRenderingContext2D tests of hibernation as needed to
  reflect the semantic change

Bug: 352263194
Change-Id: Ie6447c3a654b6a67c84640a1dcc04c33caadc73d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6781148
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1491886}