Revert of base::Time should correctly handle -epoch time values from Javascript (patchset #1 id:1 of https://codereview.chromium.org/2593073002/ )

Reason for revert:
The patched broke EME player which was relying on null time being converted to 0 in FromJsTime.

After discussing this, we agreed to try to change all conversion methods together to ensure consistency. A precursor to doing this will be to get stop treating zero as null.

For more info see:  https://bugs.chromium.org/p/chromium/issues/detail?id=679079#c13

Original issue's description:
> 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
> incorrect.
>
> * 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
>
> BUG=625680
>
> Committed: https://crrev.com/6650abf280f2987af809b0111ad00e5dba5723e4
> Cr-Commit-Position: refs/heads/master@{#440304}

TBR=miu@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=625680

Review-Url: https://codereview.chromium.org/2655233003
Cr-Commit-Position: refs/heads/master@{#446340}