Bug 1050083 - Changes to WebM buffered range handling.

Include the frame duration in the range end.  Also handle (ignore) invalid
negative timestamps when parsing.  Include cluster offset in time/data
offset record.  Cluster offset will be used when calculating discard/resync
points in MSE.
Bug 1050099 - Avoid calling deprecated FindStartTime and instead use buffer's decoded ranges in switching logic.
Bug 1050652 - Implement GetEvictionOffset for WebM reader.
WIP: Improve buffered range calculation for MediaSource objects.