RFR: 8253938: ZGC: Clean up argument names after JDK-8253030

[Albert Mingkun Yang]

On Fri, 2 Oct 2020 15:37:59 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> JDK-8253030 changed some variable names from timeout_in_millis to timeout_in_micros, but missed to do the same rename
>> in the header file. This trivial patch renames two arguments in zMark.hpp accordingly.
>
> Lgtm and trivial to me.

Maybe it makes sense to use microseconds in the destructor of `ZMarkTimeout` as well, now that all times in this file
are converting from ms to us.
    log_debug(gc, marking)("Mark With Timeout (%s): %s, " UINT64_FORMAT " oops, %.3fms",
                           ZThread::name(), _expired ? "Expired" : "Completed",
                           _check_count, TimeHelper::counter_to_millis(duration.value()));

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PR: https://git.openjdk.java.net/jdk/pull/483