Remove setToNodesInDocumentTreeScope().

No one calls this since I removed ConfusingAndOftenMisusedDisallowShadowContent.

Review URL: https://codereview.chromium.org/394123002

git-svn-id: svn://svn.chromium.org/blink/trunk@178232 bbb929c8-8fbe-4397-9dbb-9b2b20218538