Make the concept of attached implicit

Remove the need to call lazyAttach entirely. Now once you're in
the tree you're lazy attached.

This is the last major step in the lazy attach project. All that
remains after this is renaming the methods to something better
like lazyAttachIfAttached to scheduleRenderTreeCreation and
attach/detach to createRenderTree and destroyRenderTree.

BUG=299011

Review URL: https://codereview.chromium.org/66053002

git-svn-id: svn://svn.chromium.org/blink/trunk@161999 bbb929c8-8fbe-4397-9dbb-9b2b20218538