Remove setFooListener() APIs from Platform.

Those methods should no longer be used by the content layer. It should
be using startListening() and stopListening() instead.

Requires https://codereview.chromium.org/421293010/

BUG=400158

Review URL: https://codereview.chromium.org/437163005

git-svn-id: svn://svn.chromium.org/blink/trunk@179772 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove WebMediaPlayerClient::preload() and volume()

They are unused as of Chromium r288080.

The WebMediaPlayerClientImpl::m_preload member became unused, so remove
it. This makes WebMediaPlayerClientImpl::setPreload look fragile, and
it's possible that there are existing bugs where the preload state is
not propagated correctly. However, HTMLMediaElement should eventually
call WebMediaPlayer::setPreload() directly, so the removed code will be
of no use.

BUG=401415

Review URL: https://codereview.chromium.org/455453002

git-svn-id: svn://svn.chromium.org/blink/trunk@179773 bbb929c8-8fbe-4397-9dbb-9b2b20218538