[Blink] Clean up CanvasResourceProviderSI constructor

* Instead of creating an entire adjusted SkImageInfo, explicitly adjust
  only the SkColorType (the only field that changes)
* Create a gfx::Size from the SkImageInfo in a local var and use that
  through the method

No functional changes, just cleanup.

Bug: 371227617
Change-Id: I5454207f5b42a4ffc9c8f5b5a4b27c3c0b4d2874
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6055159
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1389661}