Allow callers of TimeSource::GetWallClockTime() to suspend time.

Allow callers of TimeSource::GetWallClockTime() to suspend time.

[for discussion only currently]

Adds acquire, release, and locked semantics to ensure time monotonicity
between calls while converting multiple timestamps.

GetWallClockTime() now takes a flags value which can be specified as:
- SINGLE_TIMESTAMP (acquire + release in the same call)
- SUSPEND_TIME (acquire, release in a later call)
- RESUME_TIME (acquired previously, release in this call)
- TIME_IS_SUSPENDED (acquired previously, do not release this call).

A little hairy, but the other option is to pass a vector<base::TimeDelta> in
and receive a vector<base::TimeTicks> out; which seems a bit overweight.

BUG=485042
TEST=TBD if we end up going this route.

[DaleCurtis, 2015-05-09 21:40:48]

Patchset #1 (id:1) has been deleted