We used to call toString as a method which is not safe.

BUG=v8:4225
LOG=Y
R=adamk, littledan
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_rel_ng;tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1200033003

Cr-Commit-Position: refs/heads/master@{#29242}