We're going to use HangDetails as the type containing hang information. We'll
have a JS component which reads the data out of nsIHangDetails, builds the
payload, and submits it to telemetry for us.
We'll do it in JS because telemetry has to be submitted from JS.
This patch also adds IPC serization for the relevant types so that we can send
HangDetails objects over IPDL.
MozReview-Commit-ID: CeikKabY9Vs
