Before this we were using + which calls valueOf which is not correct
for these methods.

BUG=v8:4222
LOG=N
R=adamk, littledan
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_rel_ng;tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1194173004

Cr-Commit-Position: refs/heads/master@{#29202}