Convert didAddShadowRoot() into willAddFirstAuthorShadowRoot()

There's no reason to have this hook on Element that is called whenever
any ShadowRoot is added. Instead all the users just want to know when
the first Author shadow root is added since they need to switch back
to a different rendering.

Review URL: https://codereview.chromium.org/229313004

git-svn-id: svn://svn.chromium.org/blink/trunk@171182 bbb929c8-8fbe-4397-9dbb-9b2b20218538