Merge 143765 - Ensure media's buffered ranges always have a range that includes currentTime.

Merge 143765 - Ensure media's buffered ranges always have a range that includes currentTime.
Avoids buffering bar disappearing/reappearing when the bytes are distributed
unevenly throughout the media.

THIS IS A MANUAL (not drover'd) MERGE.
Due to preceding *unmerged* changes (esp r143106) to
chunk_demuxer{,_unittest}.cc, the original CL doesn't apply cleanly to the 1180
branch.  Instead these two files are omitted from the merge, and only enough
bits are changed in chunk_demuxer.cc to allow compilation to proceed.  Also,
ranges.h needed IntersectionWith from the above unmerged CL added manually.

BUG=133567, 131444
TBR=fischman@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=145966