Speculative crash-fix for SnooperNode::Render().

The current theory (see bug) is that somehow the member set between
audio::LoopbackStream and media::LoopbackGroupCoordinator became out-of-
sync. This change tests that theory by modifying the destructor to
ensure the exact set of SnooperNode instances that could be in-use by
LoopbackStream::FlowNetwork are cleanly "disconnected" before shutdown
(with further object destruction) proceeds.

Bug: 888478, 867886
Change-Id: I7b34fe168a80302c43a4333b9a181342f7b79912
Reviewed-on: https://chromium-review.googlesource.com/c/1266875
Commit-Queue: Max Morin <maxmorin@chromium.org>
Reviewed-by: Max Morin <maxmorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597573}

services/audio/loopback_stream.cc

@@ -108,9 +108,8 @@ LoopbackStream::~LoopbackStream() {
   if (network_) {
     if (network_->is_started()) {
       coordinator_->RemoveObserver(group_id_, this);
-      for (LoopbackGroupMember* member :
-           coordinator_->GetCurrentMembers(group_id_)) {
-        OnMemberLeftGroup(member);
+      while (!snoopers_.empty()) {
+        OnMemberLeftGroup(snoopers_.begin()->first);
       }
     }
     DCHECK(snoopers_.empty());