Add fast path for setting length

Add fast path for setting array.length

BUG=

Committed: https://crrev.com/a803254413faf817785c9fc86db89c5d858223de
Cr-Commit-Position: refs/heads/master@{#29154}

[Toon Verwaest, 2015-06-19 14:08:47]

verwaest@chromium.org changed reviewers:
+ jkummerow@chromium.org

[Toon Verwaest, 2015-06-19 14:08:48]

ptal

[Jakob Kummerow, 2015-06-19 14:56:03]

lgtm

[Toon Verwaest, 2015-06-19 15:00:50]

The CQ bit was checked by verwaest@chromium.org

[commit-bot: I haz the power, 2015-06-19 15:01:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1195823002/60001

[commit-bot: I haz the power, 2015-06-19 15:27:46]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-06-19 15:27:59]

Patchset 4 (id:??) landed as
https://crrev.com/a803254413faf817785c9fc86db89c5d858223de
Cr-Commit-Position: refs/heads/master@{#29154}

[Toon Verwaest, 2015-06-19 20:48:57]

https://codereview.chromium.org/1195823002/diff/60001/src/accessors.cc
File src/accessors.cc (right):

https://codereview.chromium.org/1195823002/diff/60001/src/accessors.cc#newcod...
src/accessors.cc:205: return wrapper->value()->ToArrayIndex(length);
I think this is actually wrong...

Number.prototype.valueOf = function() { return 100; }
a = [];
a.length = new Number(10)
should probably result in an array of size 100, not 10 as it does right now.

[Jakob Kummerow, 2015-06-20 11:26:38]

https://codereview.chromium.org/1195823002/diff/60001/src/accessors.cc
File src/accessors.cc (right):

https://codereview.chromium.org/1195823002/diff/60001/src/accessors.cc#newcod...
src/accessors.cc:205: return wrapper->value()->ToArrayIndex(length);
On 2015/06/19 20:48:57, Toon Verwaest wrote:
> I think this is actually wrong...
> 
> Number.prototype.valueOf = function() { return 100; }
> a = [];
> a.length = new Number(10)
> should probably result in an array of size 100, not 10 as it does right now.

Good point, in addition to checking the wrapper's map for being pristine, we
also need to check its prototype.