[Blink] Move Canvas2d away from checking presence of C2DLayerBridge

The Canvas2D code currently uses the presence of the Canvas2DLayerBridge
to make decisions on which codepaths to take:

* Canvas2DRenderingContext::IsPaintable() returns true iff the canvas
  and its bridge exist

This CL changes CanvasRenderingContext2D::IsPaintable() to return
whether the canvas and its resource provider exist, which aligns with
OffscreenCanvasRenderingContext2D and with the intended semantics of
this method. After an audit of the callsites of this method, we also
adjust the callsites for which a straight-up call of IsPaintable()
no longer makes sense.

As this change might have unintended behavioral impact, we make it
under a killswitch.

Change-Id: I340385145b0c75f423a6f9cfee1a4ab5e41e2ae1
Bug: 40280152
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6196595
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Jean-Philippe Gravel <jpgravel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1422532}