This patch fixes an obscure edge case for functions defined as the
direct body of a for-of/for-in loop, such as the following:

 for (foo in []) function foo() { return foo; }

Here, the first occurrence of foo should point to the outer scope;
however, before this patch, it pointed to the inner foo in an
invalid way which caused an assertion about the scope chain to fail.
This patch fixes the scope chain by inserting an extra scope for
the body of the loop, not including the header.

BUG=chromium:542099
LOG=N
R=rossberg

Review URL: https://codereview.chromium.org/1396663004

Cr-Commit-Position: refs/heads/master@{#31268}