Bug 854785 - Ensure to round seconds when updating call duration

When computing the duration, glitches can interfere thus that the
duration computed might be a bit under the interval of 1000ms. In the
event of this case, one would see a first tick with, for example, a
call duration of 28.001 secs, and a next tick with a duration of 28.999.
Merge pull request #10699 from lissyx/bug854785

Bug 854785 - Ensure to round seconds when updating call duration r=etienne