[Blink] Change HTMLCanvasElement to call CanvasRC2D::GetImage()

This CL changes HTMLCanvasElement away from calling
Canvas2DLayerBridge::NewImageSnapshot() to calling the wrapper method
CanvasRenderingContext2D::GetImage(). The two calls are functionally
equivalent, as:

* `context_` is guaranteed to exist and be a CanvasRenderingContext2D if
  the bridge exists: as HTMLCanvasElement creates the bridge only in
  response to calls that originate either directly from
  CanvasRenderingContext2D or from BaseRenderingContext2D, which must
  itself be a CanvasRenderingContext2D in the context of
  HTMLCanvasElement as HTMLCanvasElement does not create
  OffscreenCanvasRenderingContext2D objects
* The only extra code that CanvasRenderingContext2D::GetImage() has
  over Canvas2DLayerBridge::NewImageSnapshot() is that it returns
  nullptr if IsPaintable() is false.
  CanvasRenderingContext2D::IsPaintable() returns whether the bridge
  exists, which by definition will be true in these calling contexts.

As noted in TODOS added here, in a later stage of this refactoring we
will port this code away from directly checking for the existence of
`canvas2d_bridge_`.

Bug: 40280152
Change-Id: I53222bb0dd7fc8efa2318ac7d913b3506541ba1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5890812
Reviewed-by: Jean-Philippe Gravel <jpgravel@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1361103}