Revert 170343 "Send AXLayoutComplete even when document hasn't f..."

Temporarily reverting so we can merge the revert to M36. Will un-revert
on trunk.

> 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

TBR=dmazzoni@chromium.org

Review URL: https://codereview.chromium.org/345563006

git-svn-id: svn://svn.chromium.org/blink/trunk@176724 bbb929c8-8fbe-4397-9dbb-9b2b20218538