Merge/rename two MediaControlElements into their parent classes

MediaControlPanelMuteButtonElement/MediaControlPanelVolumeSliderElement
were the only classes that inherited
MediaControlMuteButtonElement/MediaControlVolumeSliderElement.

Remove the m_panelMuteButton->show() in MediaControls::reset(), because
there is nothing that ever hides the button in the first place.

BUG=341813

Review URL: https://codereview.chromium.org/185363011

git-svn-id: svn://svn.chromium.org/blink/trunk@168572 bbb929c8-8fbe-4397-9dbb-9b2b20218538