Add an implementation of nsIContentPermissionRequest to encapsulate requesting
permission from the user to autoplay audible media.
All documents in the tab request permission using the top level document's
origin, so the AutoplayRequest instance for a tab is stored on the top level
content window of the tab.
AutoplayRequest ensures that there's only a single prompt shown at once.
MozReview-Commit-ID: 2u3aLnEa21z
