1. using media::TimeUnit to save some typing.
2. replace TimeUnit() with TimeUnit::Zero().
3. replace TimeUnit::FromXXX(0) with TimeUnit::Zero().
4. replace TimeUnit::FromMicroseconds(std::numeric_limits<int64_t>::max()) with TimeUnit::FromInfinity().
5. replace some uses of int64_t with TimeUnit.
6. replace t > TimeUnit() with t.IsPositive().
MozReview-Commit-ID: 6hC94PXx86i
--HG--
extra : rebase_source : 1ea3b409e6ec12915f3e1a00359d6ff4152c8917
extra : intermediate-source : e31a12ad0e7a4840119036f261ed17eaaff85734
extra : source : ae07ee48000c4a52da0e4fd502b4d690ec51ce1f
f2ea37e57b