Remove getOrCreate and postNotification functions from AXObjectCache.

The goal is to hide implementation details from anything outside of the
accessibility directory; now there are more specific functions to call
that don't require understanding how the AX tree works.

The code could still use some clean up, but I'm trying to keep each
change small and self-contained and not distract from the purpose.

BUG=425013

Review URL: https://codereview.chromium.org/663293003

git-svn-id: svn://svn.chromium.org/blink/trunk@184104 bbb929c8-8fbe-4397-9dbb-9b2b20218538