MSE: Add EventHandler attributes to MSE objects

Per intent [3], and due to recent spec change [1] for spec issue [2], this change adds
EventHandler attributes for MSE objects as follows:

Attributes for MediaSource events:
* onsourceopen
* onsourceended
* onsourceclose

Attributes for SourceBuffer events:
* onupdatestart
* onupdate
* onupdateend
* onerror
* onabort

Attributes for SourceBufferList events:
* onaddsourcebuffer
* onremovesourcebuffer

[1] https://github.com/w3c/media-source/pull/75
[2] https://github.com/w3c/media-source/issues/66
[3] https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/dM_8nlXvJIE

R=mlamouri@chromium.org,rbyers@chromium.org,chcunningham@chromium.org
BUG=623678

Review-Url: https://codereview.chromium.org/1945523003
Cr-Commit-Position: refs/heads/master@{#402946}
8 files changed