Inform destination handler if execution context is gone.

When the execution context is being destroyed, inform the destination
handler of that so that we don't try to access the execution context
that is in the process of going away.

This CL only handles the specific case in bug 848306, and is a simple
fix for that issue.  This really needs to be fixed in a better way, but
that requires significant design work.

Bug: 848306
Test: Manually run repro case and see that it doesn't happen
Change-Id: I89a4d397af15a823d9c25ef814195706f7b30b21
Reviewed-on: https://chromium-review.googlesource.com/1138722
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575459}
unsigned long OfflineAudioDestinationHandler::MaxChannelCount() const {
  return channel_count_;
}

unsigned long OfflineAudioDestinationHandler::MaxChannelCount() const {
  return channel_count_;
}
