[WebGPU] only last getMappedRange() is copied back to buffer in GPUMapMode.WRITE
<rdar://problem/125033305>
radar://125003533
Pull request: https://github.com/WebKit/WebKit/pull/26138
Committed 276382@main (71b962443b70): <https://commits.webkit.org/276382@main> Reviewed commits have been landed. Closing PR #26138 and removing active labels.