Make sure the last remote track can be removed from a MediaStream generated from a PeerConnection.

This cl change MediaStream.cpp to allow removing a remote track from a MediaStream even if there are no live tracks.
The problem is that if the implementor of the remote track has set the last tracks readyState to ended, the last track can't be removed from the MediaStream.

BUG=417245
TEST= See bug report.

Review URL: https://codereview.chromium.org/600823002

git-svn-id: svn://svn.chromium.org/blink/trunk@182651 bbb929c8-8fbe-4397-9dbb-9b2b20218538