Get rid of AXComputedObjectAttributeCache

We added AXComputedObjectAttributeCache to WebKit because we wanted to
cache some expensive computations in the AX tree, but none of the other
WebKit ports wanted it.

A much simpler solution is to just store the cached value inside the
AXObject. To determine if the value is out of date, I added a
modification count that increments whenever there's a layout or DOM
mutation. We can reuse the same modification count for more cached
attributes.

BUG=425879

Review URL: https://codereview.chromium.org/670093002

git-svn-id: svn://svn.chromium.org/blink/trunk@184331 bbb929c8-8fbe-4397-9dbb-9b2b20218538