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}
MediaControlOverflowMenuButtonElement::create(MediaControls& mediaControls) {
  MediaControlOverflowMenuButtonElement* button =
      new MediaControlOverflowMenuButtonElement(mediaControls);
  button->ensureUserAgentShadowRoot();
  button->setType(InputTypeNames::button);
  button->setShadowPseudoId(
      AtomicString("-internal-media-controls-overflow-button"));
  button->setIsWanted(false);
  return button;
}

MediaControlOverflowMenuButtonElement::create(MediaControls& mediaControls) {
  MediaControlOverflowMenuButtonElement* button =
      new MediaControlOverflowMenuButtonElement(mediaControls);
  button->ensureUserAgentShadowRoot();
  button->setType(InputTypeNames::button);
  button->setShadowPseudoId(
      AtomicString("-internal-media-controls-overflow-button"));
  button->setIsWanted(false);
  return button;
}
