Make BrowserAccessibilityManager use AXTree (re-land).

Make BrowserAccessibilityManager use AXTree (re-land).

This is the final step of migrating content/*/accessibility to make
full use of the new infrastructure in ui/accessibility.

Previously, BrowserAccessibilityManager owned the tree of
BrowserAccessibility objects and handled making changes to the
tree in response to messages from the renderer. Now, AXTree
handles the updates from the renderer and notifies
BrowserAccessibilityManager (via AXTreeDelegate) when it
should create or destroy BrowserAccessibility objects, and when
it should notify a BrowserAccessibility object that it's been
modified.

This makes the serialization/unserialization logic fully tested now,
and greatly clarifies the semantics of when in the update cycle
various notifications are called (e.g., PreInitialize and PostInitialize
are now OnDataChanged and OnUpdateFinished).

BUG=316726
NOTRY=true

Review URL: https://codereview.chromium.org/234723003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@265021 0039d316-1c4b-4281-b951-d872f2087c98