[Blink] CHECK context is composited when creating CC layer in canvas

HTMLCanvasElement::GetOrCreateCcLayerIfNeeded() currently short-circuits
out if IsCompositedForCanvas2D() is false. Its one production callsite
is here [1]. That method in turn is called in production for Canvas2D
only in these two places [2][3], both of which are guarded by
CanvasRenderingContext2D::IsComposited() being true. The latter method
returning true implies that IsCompositedForCanvas2D() is also true [4],
so we can replace this early-out with a CHECK.

[1] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/modules/canvas/canvas2d/canvas_rendering_context_2d.cc;l=1007;drc=fd05369b5220d9daae714f450e099d070a841262;bpv=1;bpt=1
[2] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/html/canvas/html_canvas_element.cc;l=2187-2188;drc=fd05369b5220d9daae714f450e099d070a841262
[3] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/layout/layout_html_canvas.cc;l=84;drc=fd05369b5220d9daae714f450e099d070a841262
[4] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/modules/canvas/canvas2d/canvas_rendering_context_2d.cc;l=209;drc=fd05369b5220d9daae714f450e099d070a841262;bpv=1;bpt=1

Bug: 352263194
Change-Id: Ib53818d60a14fccac7b0bcde247dcf380c6f31e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6798013
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1494630}