Add CursorFactoryOzone for platform cursors with ozone

This factory manages PlatformCursor objects, which are either:

- The default cursor of a particular ui::Cursor type
- Image cursors (for custom web cursors & ash image cursors).

Loading an image cursor overrides the default cursor of that type,
just as with CursorLoaderX11.

BUG=252315

Review URL: https://codereview.chromium.org/187663003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255459 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255459