MSE: Experimental support for customizing live seekable range

Adds {set,clear}LiveSeekableRange methods to MediaSource IDL, with
implementations corresponding to the recent MSE spec addition of these
methods [1]. These methods are behind the MediaSourceExperimental
RuntimeEnabled flag until intent to ship is approved in a later
milestone.
Intent to implement thread is at [2].

[1] https://github.com/w3c/media-source/pull/92
[2] https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/2znjjhg7KiY

BUG=623698
TEST=passes upstream w3c/wpt mediasource-liveseekable.html test in PR at https://github.com/w3c/web-platform-tests/pull/3184
R=chcunningham@chromium.org,mlamouri@chromium.org

Review-Url: https://codereview.chromium.org/2105593002
Cr-Commit-Position: refs/heads/master@{#402519}
4 files changed