Correctly handle -epoch time values when converting from JS time to base::Time

-11644473600 seconds (which represents windows epoch time of
|1601-01-01 00:00:00 UTC|) is a valid time value in Javascript. Incidentally
this value is internally represented by 0 which is mistakenly confused with a
null time value.

FromJsTime is meant to be used to convert time values coming from Javascript
for which 0 or -epoch do not represent null values. So the extra check was

* In fact there is a comment in FromJsTime method making it clear that 0 is a
valid value but this was missed in ToJsTime method.

TEST=./base_unittests --gtest_filter=TimeTest.JsTime


Cr-Commit-Position: refs/heads/master@{#440304}
2 files changed