First step to move common accessibility code out of content.

This changelist defines new classes and types for
accessibility in ui/accessibility. Subsequent changes will
refactor code inside content/ to inherit from AXTree and
AXNode, then we can start exposing the accessibility tree
outside of content/.

In addition, we can now use these same types in Views
accessibility and share more code.

BUG=316726

Review URL: https://codereview.chromium.org/67283004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236733 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236733