[Blink] Adjust BaseRC2D::PaintToSnapshot() semantics

This CL aligns the BaseRenderingContext2D implementation of
PaintRenderingResultsToSnapshot() with that of other implementations
that ensure that the canvas contents are up to date before taking a
snapshot. This CL also changes it to return nullptr in the case where
the provider isn't valid to align with
CanvasRenderingContext2D::GetImage() semantics.

These changes will enable it to be used in a followup CL. There are
no current calls to this method for 2D canvas [1], so this CL itself
is a no-op.

[1] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/html/canvas/canvas_rendering_context.h;l=213;bpv=1;bpt=1?q=CanvasRenderingContext&ss=chromium

Bug: 352263194
Change-Id: I0a4b2b24b3029d3ad5fbdf1d74c5489ebe95885a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6802903
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1495270}