[Blink] Rename SupportsNativeMappableSIForCanvas2D()

The query is not whether native mappable SIs are *supported* for Canvas
2D but rather whether they should be *used*. Currently we choose to use
them only in a subset of the cases where they are actually supported
(and we might choose to use mappable SIs in this flow in the future in
cases where the mapping would be done via shared memory, e.g. on
Windows).

Bug: 485669935
Change-Id: Ieed6ca7fa57607a658aaf868ce5a9c49e6ebf2c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7626839
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1593775}