HTMLMediaElement::clearMediaPlayer should acquire MediaElementAudioSourceNode::lock()

Crash report: https://cluster-fuzz.appspot.com/testcase?key=5123869056696320

There is threading race between HTMLMediaElement::clearMediaPlayer and other methods which try to use HTMLMediaElement::m_player. clearMediaPlayer has to acquire a lock before clearing m_player, just like createMediaPlayer is acquiring the lock before clearing m_player.

c.f., https://chromiumcodereview.appspot.com/23691033/ is a CL that added the lock to createMediaPlayer.

BUG=320344

Review URL: https://codereview.chromium.org/88183002

git-svn-id: svn://svn.chromium.org/blink/trunk@162730 bbb929c8-8fbe-4397-9dbb-9b2b20218538