Add support to estimate file duration.

The code will estimate the file duration if the last block duration is
0. This is not totally correct, but better then what we currently have.

BUG=https://bugs.chromium.org/p/webm/issues/detail?id=1100

Change-Id: I8f81df0bd592e6f7b1925fa2637a2e09cf182742
5 files changed