[builtins] Re-add similar String wrapper optimization for StringAdd.

[builtins] Re-add similar String wrapper optimization for StringAdd.

For string wrappers (JSValue instances with [[StringData]] internal
fields), we can shortcirciut the ToPrimitive if

  (a) the {input} map matches the initial map of the String function,
  (b) the {input} [[Prototype]] is the unmodified %StringPrototype% (i.e.
      no one monkey-patched toString, @@toPrimitive or valueOf), and
  (c) the %ObjectPrototype% (i.e. the [[Prototype]] of the
      %StringPrototype%) is also unmodified, that is no one sneaked a
      @@toPrimitive into the %ObjectPrototype%.

If all these assumptions hold, we can just take the [[StringData]] value
and return it.

This just repairs a regression introduced by removing the
weird (and broken) intrinsic %_IsStringWrapperSafeForDefaultValue, which
was intendend to something similar to this, although less efficient and
wrong in the presence of @@toPrimitive. Long-term we might want to move
into the direction of having a ToPrimitiveStub that can do common cases
while staying in JavaScript land (i.e. not going to C++).

R=jarin@chromium.org
BUG=chromium:532524
LOG=n

Committed: https://crrev.com/17f598782d7db06dbd06c29c96ac9316a1a27aab
Cr-Commit-Position: refs/heads/master@{#30890}

[Benedikt Meurer, 2015-09-23 12:56:09]

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

[Benedikt Meurer, 2015-09-23 12:56:10]

Hey Jaro,

This repairs the date-format-xparb regression by adding back a simplified and
correct version of the %_IsStringWrapperSafeForDefaultValueOf magic. Long-term
we should do better on this in general, with the FrameState rewiring we could
even inline a checked case into TurboFan once we are there. :-)
Please take a look.

Thanks,
Benedikt

[Jarin, 2015-09-23 13:16:45]

lgtm.

[Benedikt Meurer, 2015-09-23 13:17:45]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-09-23 13:17:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1366563002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1366563002/20001

[commit-bot: I haz the power, 2015-09-23 13:49:13]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-09-23 13:49:26]

Patchset 2 (id:??) landed as
https://crrev.com/17f598782d7db06dbd06c29c96ac9316a1a27aab
Cr-Commit-Position: refs/heads/master@{#30890}