Implement the infrastructure necessary to send back the corresponding pipeline ID when translating a point to layer space.
Implement the infrastructure necessary to send back the corresponding
pipeline ID when translating a point to layer space.

Fixes events in browser.html.

servo/webrender,servo/webrender,servo/webrender,servo/webrender_traits,servo/webrender,servo/webrender,DDEFISHER/webrender_traits,alanamramjit/webrender,alanamramjit/webrender,servo/webrender