This commit adds the `getImmediateDominator` method to `DominatorTree` which
returns the id of the immediate dominator of the node associated with the given
id. This enables walking the dominator tree from leaves up parents all the way
to the root of the tree.
This adds the `getImmediatelyDominated` method to `DominatorTree` which takes a
node id and returns the set of each node ids for every node that is immediately
dominated by the node with the given id. The results are sorted by greatest to
least retained size. In conjunction with the `root` attribute, this can be used
to traverse the whole dominator tree.
This commit adds the DominatorTree.webidl interface, which is only exposed to
chrome JS. The interface is implemented by mozilla::devtools::DominatorTree,
which is a thin wrapper around JS::ubi::DominatorTree. This does not expose any
methods on the DominatorTree interface, those will come as follow up changesets.
