[es6] call ToString() on template substitutions

[es6] call ToString() on template substitutions

BUG=v8:3980
R=arv@chromium.org
LOG=N

Committed: https://crrev.com/6fcc22dce19d358dcf5fc43710ecc70575190bad
Cr-Commit-Position: refs/heads/master@{#27402}

[caitp (gmail), 2015-03-23 12:48:41]

PTAL --- quick fix to ensure template substitutions are ToString()'d as spec'd
(https://people.mozilla.org/~jorendorff/es6-draft.html#sec-template-literals-r...)

[caitp (gmail), 2015-03-23 21:14:06]

caitpotter88@gmail.com changed reviewers:
+ dslomov@chromium.org

[arv (Not doing code reviews), 2015-03-24 11:20:41]

LGTM

[caitp (gmail), 2015-03-24 12:22:30]

The CQ bit was checked by caitpotter88@gmail.com

[caitp (gmail), 2015-03-24 12:22:30]

The patchset sent to the CQ was uploaded after l-g-t-m from arv@chromium.org
Link to the patchset: https://codereview.chromium.org/1027183002/#ps80001
(title: "Test ToString behaviour for tagged templates + String.raw")

[commit-bot: I haz the power, 2015-03-24 12:22:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1027183002/80001

[commit-bot: I haz the power, 2015-03-24 12:43:55]

Committed patchset #5 (id:80001)

[commit-bot: I haz the power, 2015-03-24 12:44:07]

Patchset 5 (id:??) landed as
https://crrev.com/6fcc22dce19d358dcf5fc43710ecc70575190bad
Cr-Commit-Position: refs/heads/master@{#27402}