[Blink] Move HTMLCanvasElement test away from using GetOrCreateCRP()

These tests have already drawn to the canvas at the time that they call
GetOrCreateCanvasResourceProvider(), which ensures that the CRP is
created. Thus, they can just *get* the CRP. This will facilitate moving
creation of the CanvasResourceProvider into the 2d context.

Note: I'll need to loop back around in followup to remove all knowledge
of CRP from HMTLCanvasElement altogether, but that will be for a later
stage of the work.

Bug: 352263194
Change-Id: I2846ddac70cca5fbaf6bed5ec1bcbe116f406041
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6718051
Reviewed-by: Saifuddin Hitawala <hitawala@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1484790}