[motion-path] Have inset use an offset containing block rect
<rdar://problem/115068196>
Pull request: https://github.com/WebKit/WebKit/pull/17494
Committed 267866@main (f6fd650e3830): <https://commits.webkit.org/267866@main> Reviewed commits have been landed. Closing PR #17494 and removing active labels.
Committed 267815.15@safari-7617-branch (e134f782e6d4): <https://commits.webkit.org/267815.15@safari-7617-branch> Reviewed commits have been landed. Closing PR #768 and removing active labels.