We already have a debug setting to flash views with the orange outline on layout (when View.prototype.layout is called, to be exact): Bug 176389 - Web Inspector: Layout flashing for internal View.prototype.layout. While debugging the style editor, I find it useful to have a different outline color for the initial layout.
Created attachment 353331 [details] Patch
Comment on attachment 353331 [details] Patch r=me
Comment on attachment 353331 [details] Patch Clearing flags on attachment: 353331 Committed r237578: <https://trac.webkit.org/changeset/237578>
All reviewed patches have been landed. Closing bug.
<rdar://problem/45659166>