NetLog:  Use TimeTicks::UnixEpoch() to calculate timeTickOffset.
NetLog:  Use TimeTicks::UnixEpoch() to calculate timeTickOffset.

This addresses a TODO and removes an ugly constant from NetLog.  On
the down side, TimeTicks::UnixEpoch() is cached, which may result in
less accuracy (And all bets are off if Chrome is suspended to disk).

BUG=488660

Review URL: https://codereview.chromium.org/1127133008

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#330376}

Chilledheart/chromium,Chilledheart/chromium,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,Just-D/chromium-1,TheTypoMaster/chromium-crosswalk,chuan9/chromium-crosswalk,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,chuan9/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,hgl888/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Just-D/chromium-1,TheTypoMaster/chromium-crosswalk,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,chuan9/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,chuan9/chromium-crosswalk,chuan9/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Just-D/chromium-1,chuan9/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Just-D/chromium-1,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,hgl888/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Pluto-tv/chromium-crosswalk,chuan9/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,chuan9/chromium-crosswalk,Chilledheart/chromium,Just-D/chromium-1,chuan9/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,hgl888/chromium-crosswalk