[crankshaft] Add support for %_ToString.

[crankshaft] Add support for %_ToString.

Also support %_ToString in Crankshaft utilizing the ToStringStub, which
is also used in TurboFan and fullcodegen. This is necessary to repair a
regression on Octane that was introduced when switching from the hand
crafted NonStringToString/ToString magic to %_ToString (which properly
supports @@toPrimitive).

BUG=chromium:535953, v8:4307
LOG=n

Committed: https://crrev.com/6266a9d6b576ab62e8b409d6fa0437a82b884d0d
Cr-Commit-Position: refs/heads/master@{#30955}

[Benedikt Meurer, 2015-09-28 04:50:00]

Hey Jaro,

Easy starter for the week - thanks to the HCallWithDescriptor :-).
Please take a look.

Thanks,
Benedikt

[Benedikt Meurer, 2015-09-28 04:50:10]

bmeurer@chromium.org changed reviewers:
+ jarin@chromium.org

[Jarin, 2015-09-28 04:52:57]

lgtm

[Benedikt Meurer, 2015-09-28 04:53:13]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-09-28 04:53:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1373743002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1373743002/1

[commit-bot: I haz the power, 2015-09-28 05:01:16]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-28 05:01:42]

Patchset 1 (id:??) landed as
https://crrev.com/6266a9d6b576ab62e8b409d6fa0437a82b884d0d
Cr-Commit-Position: refs/heads/master@{#30955}