Bug 1109873 - Introduce the HitTestingTreeNode class. r=

This patch extracts a HitTestingTreeNode from AsyncPanZoomController. The tree
formed by these nodes is very similar in structure to the APZC tree that we had
previously. The main difference is that we can have multiple HitTestingTreeNode
instances per APZC; each HitTestingTreeNode corresponds to a different layer
with the same metrics.

This is a first step in extracting the hit-test data entirely from the
AsyncPanZoomController class and having a 1:1 mapping with the layer tree.
Bug 1109873 - Extract HitTestResult into a helper header. r=
Bug 1109873 - Move hit-testing data into HitTestingTreeNode. r=