Bug 1512287 - Don't recalculate root_clip_rect if in the root pipeline. r=gw
Bug 1512287 - Don't recalculate root_clip_rect if in the root pipeline. r=gw

The current code panics with an out-of-bounds access here if picture
caching is used outside an iframe.

Depends on D17994

Differential Revision: https://phabricator.services.mozilla.com/D17995

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/7610aa2fbc7323c16d66a2aafe70fd779ba565cb

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender