Implement MediaStreamTrack Content Hints

Implement MediaStreamTrack Content Hints
https://bugs.webkit.org/show_bug.cgi?id=189262
<rdar://problem/44101773>

Patch by YUHAN WU <yuhan_wu@apple.com> on 2018-09-25
Reviewed by Youenn Fablet.

LayoutTests/imported/w3c:

* web-platform-tests/mst-content-hint/MediaStreamTrack-contentHint-expected.txt:

Source/WebCore:

contentHint is a new attribute which is stored in MediaStreamTrackPrivate.
https://w3c.github.io/mst-content-hint/

Covered by tests:
LayoutTests/imported/w3c/web-platform-tests/mst-content-hint/MediaStreamTrack-contentHint.html
LayoutTests/imported/w3c/web-platform-tests/mst-content-hint/idlharness.window.html

* Modules/mediastream/MediaStreamTrack.cpp:
(WebCore::MediaStreamTrack::contentHint const):
(WebCore::MediaStreamTrack::setContentHint):
* Modules/mediastream/MediaStreamTrack.h:
* Modules/mediastream/MediaStreamTrack.idl:
* platform/mediastream/MediaStreamTrackPrivate.cpp:
(WebCore::MediaStreamTrackPrivate::setContentHint):
(WebCore::MediaStreamTrackPrivate::clone):
* platform/mediastream/MediaStreamTrackPrivate.h:
(WebCore::MediaStreamTrackPrivate::contentHint):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@236478 268f45cc-cd09-0410-ab3c-d52691b4dbfc