Send AXLayoutComplete even when document hasn't finished loading yet (reland)

Because these notifications only got sent if the corresponding AXObject
already existed, they were effectively never sent before the first
load complete notification. This change creates the AXObject for the
document element after the first layout, allowing future notifications too.

BUG=353067

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170085

Review URL: https://codereview.chromium.org/199693003

git-svn-id: svn://svn.chromium.org/blink/trunk@170343 bbb929c8-8fbe-4397-9dbb-9b2b20218538