[heap] Cleanup string factory methods a bit.

[heap] Cleanup string factory methods a bit.

This is mostly removing dead code and also dropping MUST_USE_RESULT
annotations from methods that cannot throw an exception anyways.

R=hpayer@chromium.org

Committed: https://crrev.com/3bb0174330c7eca4ebb80670652249b1aaa0e2b3
Cr-Commit-Position: refs/heads/master@{#30995}

[Hannes Payer (out of office), 2015-09-28 16:14:20]

lgtm

[Michael Starzinger, 2015-09-28 16:42:24]

I have just one question. Why are we adding dead code? :(

[Michael Starzinger, 2015-09-29 07:50:49]

The CQ bit was checked by mstarzinger@chromium.org

[Michael Starzinger, 2015-09-29 07:50:49]

The patchset sent to the CQ was uploaded after l-g-t-m from hpayer@chromium.org
Link to the patchset: https://codereview.chromium.org/1370153002/#ps20001
(title: "Remove dead test.")

[commit-bot: I haz the power, 2015-09-29 07:50:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1370153002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1370153002/20001

[commit-bot: I haz the power, 2015-09-29 08:08:21]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-09-29 08:08:37]

Patchset 2 (id:??) landed as
https://crrev.com/3bb0174330c7eca4ebb80670652249b1aaa0e2b3
Cr-Commit-Position: refs/heads/master@{#30995}