Add BandwidthEstimator to estimate the required bandwidth for the
segments.
Also includes unit test for the change.
Change-Id: I28262424e2ed6ceebdf81e2b11dcd67feba1d68c
The tests are similar because MpdWriter uses MpdBuilder.
Also changing tests and enabling them in mpd_builder_unittest.cc.
Change-Id: I76a8cfca27c390a039de765231c76d083c3ca26b