Rename ShadowRoot::insertionPoint() to ShadowRoot::shadowInsertionPointOfYoungerShadowRoot() and make it more typesafe as well.

BUG=None
Test=None; No behaviour change;

Review URL: https://codereview.chromium.org/51763005

git-svn-id: svn://svn.chromium.org/blink/trunk@161010 bbb929c8-8fbe-4397-9dbb-9b2b20218538