In the JSCallReducer we'd inline certain builtins like the Array
constructor or Function builtins across native contexts, which at
this point should be mostly safe, but might lead to cross context
leaks in the future (as it's not obvious that the JSCallReducer)
doesn't maintain this invariant. So better safe than sorry.

R=yangguo@chromium.org
BUG=v8:5267

Review-Url: https://codereview.chromium.org/2651133002
Cr-Commit-Position: refs/heads/master@{#42643}