Bug 887586 - Replace referece to current accessible in VisualPresenter with a WeakMap. r=davidb

This patch does other things to:
1. Refactor out getBounds()
2. Fix a mistake in PivotContext._isDefunct()
Bug 887592 - Fix filtering on browser events. r=davidb
Bug 775459 - Make APZC attachable only to container layers. r=kats

--HG--
extra : rebase_source : 5a94d9958013f92e64db4781e09bdc9db4b66c9a
Bug 775459 - Implement hit testing on layer tree + tests. r=kats

--HG--
extra : rebase_source : c596cc4c4dd9810319830ec8105a5e251e9b0ddb