Revert "Keep AudioHandlers alive until they can be safely deleted."

This reverts commit 071df33edf2c8b4375fa432a83953359f93ea9e4.

Reason for revert:
This CL seems to cause an AudioNode leak on the Linux leak bot.
The log is:
https://ci.chromium.org/buildbot/chromium.webkit/WebKit%20Linux%20Trusty%20Leak/14252
* webaudio/AudioNode/audionode-connect-method-chaining.html
* webaudio/Panner/pannernode-basic.html
* webaudio/dom-exceptions.html

Original change's description:
> Keep AudioHandlers alive until they can be safely deleted.
> 
> When an AudioNode is disposed, the handler is also disposed.  But add
> the handler to the orphan list so that the handler stays alive until
> the context can safely delete it.  If we don't do this, the handler
> may get deleted while the audio thread is processing the handler (due
> to, say, channel count changes and such).
> 
> For an realtime context, always save the handler just in case the
> audio thread is running after the context is marked as closed (because
> the audio thread doesn't instantly stop when requested).
> 
> For an offline context, only need to do this when the context is
> running because the context is guaranteed to be stopped if we're not
> in the running state.  Hence, there's no possibility of deleting the
> handler while the graph is running.
> 
> This is a revert of
> https://chromium-review.googlesource.com/c/chromium/src/+/860779, with
> a fix for the leak.
> 
> Bug: 780919
> Change-Id: Ifb6b5fcf3fbc373f5779256688731245771da33c
> Reviewed-on: https://chromium-review.googlesource.com/862723
> Reviewed-by: Hongchan Choi <hongchan@chromium.org>
> Commit-Queue: Raymond Toy <rtoy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#528829}

TBR=rtoy@chromium.org,hongchan@chromium.org

Change-Id: Ibf406bf6ed34ea1f03e86a64a1e5ba6de0970c6f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 780919
Reviewed-on: https://chromium-review.googlesource.com/863402
Reviewed-by: Taiju Tsuiki <tzik@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528888}
String AudioHandler::GetChannelCountMode() {
  switch (new_channel_count_mode_) {
    case kMax:
      return "max";
    case kClampedMax:
      return "clamped-max";
    case kExplicit:
      return "explicit";
  }
  NOTREACHED();
  return "";
}

String AudioHandler::GetChannelCountMode() {
  switch (new_channel_count_mode_) {
    case kMax:
      return "max";
    case kClampedMax:
      return "clamped-max";
    case kExplicit:
      return "explicit";
  }
  NOTREACHED();
  return "";
}
