This patch includes:
- (By Yoric) Don't collect/save the session when the user is idle;r=mdeboer
- Add a test for the behavior of state writing in idle/active mode

When the user is not actively using the computer, webpages may still
perform changes that require (re)writing to sessionstore, e.g.  updating
Session Cookies or DOM Session Storage, or refreshing, etc. Before
this patch, a single active page can require us to
recollect/serialize/write the entire Session Restore file every 15
seconds even when the user is not in front of the computer.

We expect that, when the user is not in front of the computer, changes
are not critical and don't need to be saved as often. We now adopt the
following strategy:

- when the user has been away for (by default) 15 seconds, finish any
  pending collect/write, then increase the collect/write buffering
  delay to (by default) 1h

- when the user returns, reschedule any pending 1h collect/write as a
  (by default) 15 seconds collect/write, then proceed with (by
  default) 15 seconds collect/write delays.

--HG--
extra : histedit_source : b7ea6a6fbfee2f3a2bddeaa69b6446d7544c2585