AX: Calendar Picker: Initial day selection should notify ActiveDescendantChanged a11y event.

AXRenderObject assumes that both of the ancestor element and the active
descendant element have their renderers.

BUG=123896

Review URL: https://codereview.chromium.org/550853002

git-svn-id: svn://svn.chromium.org/blink/trunk@181547 bbb929c8-8fbe-4397-9dbb-9b2b20218538