[ChromeDriver] Use absolute time in log file

Modify ChromeDriver log file to use absolute time (number of seconds
from Unix Epoch) instead of relative time (number of seconds since
ChromeDriver start). This makes it easier to correlate the timestamps
against event recorded elsewhere. The resolution of the timestamps
remain at milliseconds.

Bug: chromedriver:2179
Change-Id: I7d52eeb2bb90204850d04e36a9ec74560fc56834
Reviewed-on: https://chromium-review.googlesource.com/830530Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524732}

chrome/test/chromedriver/logging.cc

@@ -112,7 +112,7 @@ bool HandleLogMessage(int severity,
     std::string entry = base::StringPrintf(
         "[%.3lf][%s]: %s",
         base::TimeDelta(base::TimeTicks::Now() -
-                        base::TimeTicks::FromInternalValue(g_start_time))
+                        base::TimeTicks::UnixEpoch())
             .InSecondsF(),
         level_name,
         message.c_str());