Avoid unnecessary work when initializing a LifecycleObserver<>.

When m_lifecycleContext is not 0, the subsequent call to observeContent() will try to unregister "this" from m_lifecycleContext, which is unnecessary.

Review URL: https://codereview.chromium.org/264733004

git-svn-id: svn://svn.chromium.org/blink/trunk@173052 bbb929c8-8fbe-4397-9dbb-9b2b20218538