Fix ScopeInfo::StackSlotIndex to skip over non-locals

Fix ScopeInfo::StackSlotIndex to skip over non-locals

Block scoping can cause lexical variables in inner scopes to be allocated
stack slots in their outer declaration scope. Neither of the callers of this
method expected such variables to be returned, so it has been changed to
only search the portion of the stack slots that correspond
to locals. Also renamed the method to match the new behavior.

This method only has two callers, one a DCHECK in scopes.cc and the other
the accessor for Function.arguments. I have been unable to reproduce the
former case (though the old code definitely wasn't correct, from inspection).
For the latter, it's not currently exercisable, given that Function.arguments
does not exist in strict mode and V8 does not yet implement block scoping in
sloppy mode.

BUG=v8:4259
LOG=n

[adamk, 2015-07-01 22:23:09]

adamk@chromium.org changed reviewers:
+ rossberg@chromium.org

[adamk, 2015-07-01 22:23:16]

The CQ bit was checked by adamk@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-01 22:23:27]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1215423002/1

[adamk, 2015-07-01 22:27:36]

Ignore this for now, I realize it's not yet correct.

[adamk, 2015-07-01 23:09:06]

Sorry for the noise, closing.