Slots in ConsString/SlicedString can point to an evacutaion candidate.
The MakeExternal function makes in-place conversion to external string.
After the conversion we can have a recorded slot containing an external
pointer. As long as the external pointer is aligned, this is not a
problem. We clear the recorded slots to fix verify-heap checks.

BUG=chromium:631969
LOG=NO

Finalizing CL: https://codereview.chromium.org/2199863002/

Review-Url: https://codereview.chromium.org/2242183003
Cr-Commit-Position: refs/heads/master@{#38653}