Remove mute and volume state from WebMediaPlayerClientImpl

Removed the m_volume and m_muted state in WebMediaPlayerClientImpl and
fixed a few minor bugs caused by this state being inconsistantly
updated between HTMLMediaElement and WebMediaPlayerClientImpl.

BUG=350571
TEST=All layout tests still pass.

Review URL: https://codereview.chromium.org/191673002

git-svn-id: svn://svn.chromium.org/blink/trunk@168856 bbb929c8-8fbe-4397-9dbb-9b2b20218538