[Blink>Media] Allow autoplay muted on Android by default

There was a mistake causing autoplay muted is shipped on Android
but it will be disabled if the chromium embedder doesn't specify
content setting for "AllowAutoplay" preference. This CL makes the
AllowAutoplay preference true by default so that it is allowed by
embedders (including AndroidWebView) unless they explicitly
disable it.

Intent to ship:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/Q1cnzNI2GpI/AL_eyUNABgAJ

BUG=689018

Review-Url: https://codereview.chromium.org/2677173002
Cr-Commit-Position: refs/heads/master@{#448423}
void HTMLMediaElement::forgetResourceSpecificTracks() {
  if (m_textTracks) {
    TrackDisplayUpdateScope scope(this->cueTimeline());
    m_textTracks->removeAllInbandTracks();
  }

  m_audioTracks->removeAll();
  m_videoTracks->removeAll();

  m_audioTracksTimer.stop();
}

void HTMLMediaElement::forgetResourceSpecificTracks() {
  if (m_textTracks) {
    TrackDisplayUpdateScope scope(this->cueTimeline());
    m_textTracks->removeAllInbandTracks();
  }

  m_audioTracks->removeAll();
  m_videoTracks->removeAll();

  m_audioTracksTimer.stop();
}
