Revert 160798 "Avoid leaking MessagePortChannels when ports are ..."

I think it may have broken content_browsertests
http://build.chromium.org/p/chromium.linux/builders/Linux%20Aura/builds/10913/steps/content_browsertests/logs/stdio

> Avoid leaking MessagePortChannels when ports are cloned
> 
> MessagePortChannel used to hold a RefPtr to its entangled pair for the
> sole purpose of answering MessagePortChannel::isConnectedTo(MessagePort*).
> But answering that question only requires a pointer to the MessagePort, not
> the channel.
> 
> This patch switches MessagePortChannel back to being non-refcounted, as it
> was back when PlatformMessagePortChannel still existed. Neutered MessagePorts
> thus delete their MessagePortChannel rather than simply deref()-ing them.
> 
> In the long run, getting rid of MessagePortChannel altogether and having
> MessagePort directly interact with WebMessagePortChannel is probably the
> right thing, but an incremental approach is likely the right route.
> 
> R=abarth
> BUG=311229
> 
> Review URL: https://codereview.chromium.org/48793004

TBR=adamk@chromium.org

Review URL: https://codereview.chromium.org/51063002

git-svn-id: svn://svn.chromium.org/blink/trunk@160846 bbb929c8-8fbe-4397-9dbb-9b2b20218538