Make ChannelInterface::Enabled() be async.

* Changing return value from bool to void since the operation as async
  effects anyway.
* Removing the `enabled()` accessor due to potential threading issues
  and potential TOCTOU issues. It was only used in one place anyway.
* Applying thread restrictions to member variables.

Bug: none
Change-Id: I51949f5594339952d7b717cfd82f99b532e86b23
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/216182
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33830}

pc/channel_interface.h

@@ -37,10 +37,8 @@ class ChannelInterface {
 
   virtual const std::string& content_name() const = 0;
 
-  virtual bool enabled() const = 0;
-
   // Enables or disables this channel
-  virtual bool Enable(bool enable) = 0;
+  virtual void Enable(bool enable) = 0;
 
   // Used for latency measurements.
   virtual sigslot::signal1<ChannelInterface*>& SignalFirstPacketReceived() = 0;