Fix detection of indexed properties in Object.defineProperty()

Fix detection of indexed properties in Object.defineProperty()

When defining an indexed property on an Array object, the object's
length property should (perhaps) be updated.  This was done for any
property for which

  ToUInt32(name) == ToNumber(name)

was true, meaning any property name that, when converted to a number,
was an integer in the range [0, 2^32).  The detection should be more
strict; an indexed property is one for which

  ToString(ToUInt32(name)) == name

is true only.

Review URL: https://codereview.chromium.org/13914003

Patch from Jens Lindström <jl@opera.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14242 ce2b1a6d-e550-0410-aec6-3dcde31c8c00