Implement Array::concat function in C++.

Implement Array::concat function in C++.

The performance of Array::concat is critical of jQuery benchmark from
http://www.dromaeo.com. Our current implementation in JavaScript is very
generic and is several times slower than JSC and SpiderMonkey.

Re-implement Array::concat in C++ to take advantage of underlying implementation
details. This cuts dom-travesal-jquery execution time by half.

We may want to move Array specific implementation into a separate source file,
say jsarray.cc.

Review URL: http://codereview.chromium.org/7990

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@625 ce2b1a6d-e550-0410-aec6-3dcde31c8c00