Encapsulate PlatformMessagePortChannel a little better yet
https://bugs.webkit.org/show_bug.cgi?id=112479

Reviewed by Sam Weinig.

* dom/MessagePortChannel.h: We no longer expose PlatformMessagePortChannel in
public functions in cross-platform code.

* dom/default/PlatformMessagePortChannel.cpp:
(WebCore::MessagePortChannel::createChannel): Tweaked the factory method to avoid
using the old create() function.
(WebCore::MessagePortChannel::~MessagePortChannel): Removed an unhelpful comment.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146030 268f45cc-cd09-0410-ab3c-d52691b4dbfc