Make Frame use an index, not an iterator.

This avoids the possibility of iterator invalidation when stk grows.

Fixes #191.

Change-Id: Id51f468ff6b5f783b844b3eaaf55c1c3cc10220d
Reviewed-on: https://code-review.googlesource.com/c/37190
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed