Do not update attributes in HTMLBodyElement::insertedInto.

Use didNotifySubtreeInsertionsToDocument instead.

BUG=356095
TEST=automated
R=morrita@chromium.org

Review URL: https://codereview.chromium.org/212793007

git-svn-id: svn://svn.chromium.org/blink/trunk@170216 bbb929c8-8fbe-4397-9dbb-9b2b20218538