This patch implements async returns for IPDL using MozPromises. There
are following changes:
* Initialize AbstractThreads for MessageLoops
* Record promises and their reject functions
* When async message returns, call their resolve functions
* When send error or channel close, call their reject functions
* Implement "unresolved-ipc-promises" count for about:memory
* Test cases
See bug attachment for generated code from test cases
MozReview-Commit-ID: 7xmg8gwDGaW
--HG--
rename : ipc/ipdl/test/ipdl/error/AsyncReturn.ipdl => ipc/ipdl/test/ipdl/ok/AsyncReturn.ipdl
extra : rebase_source : 9a5821d6c0e5f7152b8152a17a409b94e8258dc3