Codify the assumption that %GetArrayKeys can return only a single interval starting at zero

This patch adds comments explaining the interface in runtime.cc and simplifies
all callers given these assumptions (e.g., no need to loop over intervals, or
calculate where the interval starts).

Took care of some unrelated issues in the edited code:
- Fixes one use of [] to InternalArray
- Removed a bunch of comments referring to ES3 which no longer hold in ES5

Review URL: https://codereview.chromium.org/13071006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14125 ce2b1a6d-e550-0410-aec6-3dcde31c8c00