This adds code to allow Shaka Player to play media in sequence
mode, an alternate playback mode that makes the browser ignore
media timestamps, when playing HLS media.
This is important for containerless media formats, as they do not
contain such timestamps.
Changing HLS to not require timestamps also means that we no
longer need to fetch media segments in order to get the start
time, which should lower bandwidth usage and startup delay.
In initial tests, on a simulated 3G network, load latency went down
from an average 3.16s to 2.61s on the HLS version of "Big Buck Bunny:
the Dark Truths of a Video Dev Cartoon"; an improvement of about 17%.
Issue #2337
Change-Id: I507898d74ae30ddfb1bddf8dce643780949fbd9b
Michelle Zhuo