Move some AX attrs from AXNodeData to AXTreeData.

Move some AX attrs from AXNodeData to AXTreeData.

Previously all accessibility attributes had to be stored
in AXNodeData. The root of a tree had a lot of extra
attributes that only applied to that node. This introduces
AXTreeData, a simple serializable struct with the data
that applies to the whole tree, including the title, url,
loading progress, and selection information. Later it will
include focus information too.

BUG=none