It doesn't do anything on legacy iOS, AFAICT. If it was supposed to be shipped with iOS 8, then this bug should be changed to "paint flashing is broken when remote inspecting iOS 8".
<rdar://problem/18602659>
This should just be removing "PageAgent.setShowPaintRects" from the Legacy Inspector JSON files. They had the method, but they didn't implement it. So they are really falsely reporting that they support it.
Created attachment 239767 [details] [PATCH] Proposed Fix
Created attachment 239809 [details] [PATCH] For Bots
Comment on attachment 239809 [details] [PATCH] For Bots Clearing flags on attachment: 239809 Committed r174696: <http://trac.webkit.org/changeset/174696>
All reviewed patches have been landed. Closing bug.