Document::attach doesn't need to call recalcStyle

When the document is being attached we know there's no style resolver,
no documentElement and no plugins, so we can do something much simpler
than calling recalcStyle.

Review URL: https://codereview.chromium.org/27138002

git-svn-id: svn://svn.chromium.org/blink/trunk@159624 bbb929c8-8fbe-4397-9dbb-9b2b20218538