Bug 838625 - Add logcat logging to ProcessPriorityManager. r=cjones

The logging is placed behind an #if 0 guard.

--HG--
extra : rebase_source : 9105e7225a2e64138c4d6fb2ea62209fc7f29ca2
Bug 838625 - Add hal::ProcessPriorityToString and use it in ProcessPriorityManager logging. r=cjones

--HG--
extra : rebase_source : 0a9ecf848a9c6c3ba37acafa24c775154ccb417f
Bug 838620 - Eliminate the ChildOSPrivileges typedef in favor of base::ChildPrivileges. r=cjones

--HG--
extra : rebase_source : 9b8a8628d0e5ba75989d6d3eeacbf55a2796b27d
Bug 838616 - Boost preallocated process's priority sooner after we decide to transform it into an app process. r=cjones

--HG--
extra : rebase_source : 011d56c6e06bda680d073ae420b84d42e8a37257
Bug 838616 - Eliminate a process-priority race condition around preallocated process creation. r=cjones

Previously, it was possible to return a dead preallocated process.  This
patch eliminates or at least significantly reduces the likelihood of
this race.

--HG--
extra : rebase_source : f51022581edbdee8b72a8a56e296b6706bf1d165
Bug 838615 - Add missing process-priority nice values to b2g.js. r=cjones

This sets background_perceivable's nice at 10 and
background{,_homescreen}'s nice at 20.  I'm a bit worried that this will
de-prioritize background_perceivable so much that music players may
start skipping.  But we need to test.

--HG--
extra : rebase_source : e759ffb5bb58b89838c1dafdc49f737832593961