[//cc] Get stride from MappableSI for SW in ZeroCopyRBP

Passing `0` for the stride tells `PlaybackToMemory()` that the stride is
tightly packed [1]. For shared memory the stride is tightly packed, so
just using it directly is a behavioral no-op.

[1] https://source.chromium.org/chromium/chromium/src/+/main:cc/raster/raster_buffer_provider.cc;l=74-75?q=RasterBufferProvider::PlaybackToMemory&ss=chromium

Bug: 403372453
Change-Id: Ifad93a782b5807d68082a076a388d7b34f9e0f49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6415430
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1440740}