Store IdTargetObserver::m_registry as a reference

Store IdTargetObserver::m_registry as a reference instead of a pointer.
This makes the code look safer and gets rid of an unnecessary null check.

Also make IdTargetObserverRegistry non copyable to protect against bad
assignments to references.

R=adamk@chromium.org, adamk, rwlbuis

Review URL: https://codereview.chromium.org/183463004

git-svn-id: svn://svn.chromium.org/blink/trunk@168419 bbb929c8-8fbe-4397-9dbb-9b2b20218538