Add support for chrome devtools extension

The definitions of `chrome.devtools` API are all done in blink, so it is possible for us to provide support for chrome devtools extension without importing chrome's extension APIs.

Problems:

1. [x] Panels are loaded in `iframe`s, which are sandboxed by default in atom-shell, so panels can not access `chrome.devtools` API.
2. [x] Pages in extensions are all relative to root, so we have to provide a custom protocol to load extensions.
3. [ ] Some extensions use background page.
4. [x] Some extensions use chrome extension APIs like `chrome.extension` and `chrome.runtime`.
5. [x] Extensions information should be persistented.