Summary: Adds a new plugin to support [LeakCanary](https://github.com/square/leakcanary), displaying memory leaks as they are detected. Each leak shows a hierarchical path from the GC root to the leaked object, and allows inspection of these objects' fields.
Reviewed By: jknoxville
Differential Revision: D8865149
fbshipit-source-id: 99bcf216578b9d6660ead7d48b9bafe0d20a6c08
1008dbb283