[Blink] Make GetOrCreateCRPImpl() private method of CRCHost

Outside of CanvasRenderingContextHost, it's called only by
HTMLCanvasElement. That callsite is 2D-specific and hence can just call
CRCHost::GetOrCreateCanvasResourceProviderForCanvas2D() instead (the
latter method wraps GetOrCreateCanvasResourceProviderImpl() with a
CHECK that the context is canvas2D). This CL makes that change.

Bug: 352263194
Change-Id: Ic9b9f5a7847303fbda02b0696f7264bc15942081
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6620071
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1470396}