Back timestamp is computed by summing the new frames_written for every
call to WroteAudio. The number of microseconds per frame is often not
a whole number (e.g. 20.833 mu for sample rate of 48Khz). Prior to this
change, using TimeDelta to do the summing of frames_written meant we
truncated to the nearest microsecond with every call to WroteAudio. The
truncation error slowly accumulates in the back timestamp. After 2
hours of playback this error causes noticeable audio/video sync drift.

Having front_timestamp be a double is less critical. Front timestamp
is computed using back_timestamp at every call to WroteAudio, so fixing
back implicitly fixes front. Still, I've changed them both to double
for the sake of consistency and a slight improvement in accuracy.

BUG=564604

Review URL: https://codereview.chromium.org/1711473002

Cr-Commit-Position: refs/heads/master@{#376287}