Use of the playback time in the MSE garbage collection.

Use of the playback time in the MSE garbage collection.

The MSE spec clearly mentions what should be the state
of the media element based on the buffered ranges
(see MSE spec 2.4.4 SourceBuffer Monitoring).
Since there is a latency between the read position
in the source buffer stream and the playback position,
the MSE garbage collection must be adjusted so as to
not remove the playback position from the buffered ranges.

Conflicts:
	content/renderer/media/webmediaplayer_impl.cc
	media/filters/chunk_demuxer.cc
	media/filters/source_buffer_stream.cc

BUG=

[kjoswiak, 2014-12-09 20:58:21]

Ported from Chromecast codebase, a more up-to-date version of
https://codereview.chromium.org/341083004/

[DaleCurtis, 2014-12-09 21:14:48]

dalecurtis@chromium.org changed reviewers:
+ wolenetz@chromium.org

[DaleCurtis, 2014-12-09 21:14:58]

dalecurtis@chromium.org changed reviewers:
+ damienv@chromium.org

[damienv1, 2014-12-11 17:46:26]

As discussed in https://codereview.chromium.org/341083004/ with Aaron, we can
have a better design by moving the garbage collection outside of the append loop
(You should discuss with Mat about this).
Only the garbage collection needs the playback time (so as to be compliant with
the MSE section you mentioned).
Doing so will avoid propagating the playback time to modules that do not need
it.

[kjoswiak, 2014-12-11 23:25:26]

After giving this some thought, I am combining this CL with another I have open,
https://codereview.chromium.org/789983003/. This other CL provides an alternate
path for GC to trigger that is more in line with MSE spec, enabling removing GC
from internal append loop. However, implementing that CL doesn't make sense
until we tone down GC aggressiveness (this CL), and as noted this CL can be
simplified by not calling GC during append loop, and not having to track media
time in SourceBufferStream.

New CL: https://codereview.chromium.org/794343003/