Fixed volume slider element event handling

MediaControlVolumeSliderElement::defaultEventHandler has making
redundant calls to setVolume() & setMuted() on mouse activity. E.g. if
a mouse click changed the slider position, the above calls were made 4
times, once for each of these events: mousedown, input, mouseup,
DOMActive, click. This crack got exposed when PointerEvents are enabled
by default on M55, adding pointermove, pointerdown & pointerup to the
list.

This CL fixes the code to trigger the calls to setVolume() & setMuted()
only when the slider position is changed. Also added pointer events to
certain lists of mouse events in the code.

BUG=677900

Review-Url: https://codereview.chromium.org/2622273003
Cr-Commit-Position: refs/heads/master@{#446032}
    MediaControlTimeRemainingDisplayElement(MediaControls& mediaControls)
    : MediaControlTimeDisplayElement(mediaControls, MediaTimeRemainingDisplay) {
}

    MediaControlTimeRemainingDisplayElement(MediaControls& mediaControls)
    : MediaControlTimeDisplayElement(mediaControls, MediaTimeRemainingDisplay) {
}
