WebAudio: Remove AudioNode::RefType.

The motivation of this CL is to reduce manual ref()/deref().

AudioNode has two types of ref()/deref(); RefTypeNormal and RefTypeConnection.
It made the code complex, and we couldn't use RefPtr<> for the connection type
references.

This CL removes AudioNode::RefType.
 - m_normalRefCount and m_connectionRefCount were updated independently. After
   this CL, connection type references updates both of them.
 - We use RefPtr<> for connection type references, and call
   AudioNode::makeConnection() and AudioNode::breakConnection() explicitly when
   connections are updated.

BUG=392788
TEST=none; This CL should not have behavior changes.

Review URL: https://codereview.chromium.org/349213007

git-svn-id: svn://svn.chromium.org/blink/trunk@177910 bbb929c8-8fbe-4397-9dbb-9b2b20218538