Add a TurboFan skeleton for StringAddStub.

Add a TurboFan skeleton for StringAddStub.

Currently the stub simply calls out to the runtime, this will be
improved in a later CLs. The current state at least avoids bit-rot and
later merging horror.

Fixes frame construction logic for stubs, too, and contains quite a few
tiny cleanups in stub-land.

Committed: https://crrev.com/f91503f25587cf7fdd57edc460fb2349f9ea9c2c
Cr-Commit-Position: refs/heads/master@{#28555}

[Sven Panne, 2015-05-20 15:19:44]

svenpanne@chromium.org changed reviewers:
+ mvstanton@chromium.org

[mvstanton, 2015-05-21 08:57:27]

Hi Sven, LGTM.

[Sven Panne, 2015-05-21 14:05:59]

The CQ bit was checked by svenpanne@chromium.org

[Sven Panne, 2015-05-21 14:05:59]

The patchset sent to the CQ was uploaded after l-g-t-m from
mvstanton@chromium.org
Link to the patchset: https://codereview.chromium.org/1150673002/#ps40001
(title: "Fixed frame construction logic")

[commit-bot: I haz the power, 2015-05-21 14:06:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1150673002/40001

[commit-bot: I haz the power, 2015-05-21 14:31:49]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-05-21 14:32:09]

Patchset 3 (id:??) landed as
https://crrev.com/f91503f25587cf7fdd57edc460fb2349f9ea9c2c
Cr-Commit-Position: refs/heads/master@{#28555}