Allow to remove MediaStreamPrivate observers when iterating over observers

Allow to remove MediaStreamPrivate observers when iterating over observers
https://bugs.webkit.org/show_bug.cgi?id=187256

Reviewed by Eric Carlson.

Migrate the observer list from a Vector to a HashSet.
This is more robust to multiple observing and keeping of order of observers is not required.
Copy the set of observers to a vector before iterating over it.
This allows to remove an observer while iterating, which is now used in UserMediaRequest.

Covered by existing tests.

* Modules/mediastream/UserMediaRequest.cpp:
(WebCore::UserMediaRequest::mediaStreamIsReady):
* platform/mediastream/MediaStreamPrivate.cpp:
(WebCore::MediaStreamPrivate::addObserver):
(WebCore::MediaStreamPrivate::removeObserver):
(WebCore::MediaStreamPrivate::forEachObserver const):
(WebCore::MediaStreamPrivate::updateActiveState):
(WebCore::MediaStreamPrivate::addTrack):
(WebCore::MediaStreamPrivate::removeTrack):
(WebCore::MediaStreamPrivate::characteristicsChanged):
* platform/mediastream/MediaStreamPrivate.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@238181 268f45cc-cd09-0410-ab3c-d52691b4dbfc