Cleanup elapsed time calculation

Note: this also fixes a bug where the fractional second was incorectly
displayed.  Since the usec wasn't zero-padded, it was being left shifted:
For example, the value "6.000005" was being printed as "6.5".

BUG=chromium-os:21568
TEST=Inspect timer output.
It should always have 6 decimal places, even if usec < 100000.

Change-Id: Ia41a11428aedde45419d99821a411144156e273a
1 file changed