Fix string HTML methods to call ToString

Fix string HTML methods to call ToString

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

Committed: https://crrev.com/6b268bce6a835b0517468b4b4253b7ce2affdad8
Cr-Commit-Position: refs/heads/master@{#29202}

[arv (Not doing code reviews), 2015-06-22 17:56:17]

PTAL

[adamk, 2015-06-22 18:00:46]

lgtm

[arv (Not doing code reviews), 2015-06-22 18:16:50]

The CQ bit was checked by arv@chromium.org

[commit-bot: I haz the power, 2015-06-22 18:17:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1194173004/1

[commit-bot: I haz the power, 2015-06-22 19:07:14]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-22 19:07:32]

Patchset 1 (id:??) landed as
https://crrev.com/6b268bce6a835b0517468b4b4253b7ce2affdad8
Cr-Commit-Position: refs/heads/master@{#29202}