using newly introduced ThinStrings, which store a pointer to the actual,
internalized string they represent.

BUG=v8:4520

(Previously landed as #42168 / af51befe)
(Previously landed as #42193 / 4c699e34)

Review-Url: https://codereview.chromium.org/2549773002
Cr-Commit-Position: refs/heads/master@{#42235}