90% of our rendering time in debug builds with Skia content is spent validating the clip stack, which rebuilds it item by item every time the clip is changed at all. This is redundant since new clips are individually validated when they're added to the stack. This is causing problems with timeout on mochitests across the board.

Pushed by lsalzman@mozilla.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/efbbc635257e disable slow debug validation in Skia. r=mchang