Setting HTMLMediaElement.controller does not properly remove the 'mediagroup' content attribute

Setting HTMLMediaElement.controller does not properly remove the 'mediagroup' content attribute,
as mandated by the specification:
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-video-element.html#dom-media-controller

We had custom bindings code for the controller attribute setter which attempt to remove the
mediaGroup attribute by calling HTMLMediaElement::setMediaGroup(String()). However, while this
updates the m_mediaGroup member in HTMLMediaElement, this does not really remove the attribute.

Since mediaGroup is a reflected attribute, accessing the mediaGroup attribute from JS will end up
calling Element::fastGetAttribute(mediaGroupAttr) on native side, and bypass the value stored in
HTMLMediaElement::m_mediaGroup.

This CL updates HTMLMediaElement::setController() so that it calls removeAttribute(mediaGroupAttr)
to actually remove the attribute. It also gets rid of the custom bindings code and the m_mediaGroup
member entirely as it is bug prone.

BUG=316636
TEST=media/media-controller.html

Review URL: https://codereview.chromium.org/59233014

git-svn-id: svn://svn.chromium.org/blink/trunk@161812 bbb929c8-8fbe-4397-9dbb-9b2b20218538