AX: Calendar Picker: Apply aria-haspopup=true to PickerIndicatorElement.

It's simpler than providing its own aria-label string.
Neither a layout test nor a unit test can test this change.

BUG=123896

Review URL: https://codereview.chromium.org/572603006

git-svn-id: svn://svn.chromium.org/blink/trunk@182219 bbb929c8-8fbe-4397-9dbb-9b2b20218538