Simplify _childHashFor* functions implementation

- _childHashFor* now use global edge index as the hash value
- added type annotations to the methods

BUG=None

Review URL: https://codereview.chromium.org/224843005

git-svn-id: svn://svn.chromium.org/blink/trunk@170937 bbb929c8-8fbe-4397-9dbb-9b2b20218538