fix variable name causing syntax error in Android 2.3

In the `resolvePrototypeChain` function of `CustomElements.js`, the variable named `native` causes a syntax error when run in an Android 2.3 web browser. This is easily fixed by renaming the variable to something else.

Reproducing the issue:

- In an Android 2.3 web browser, enable JavaScript console by typing `about:debug` into the address bar, then enable the appropriate debug setting
- Visit http://jsfiddle.net/r4GFt/ and notice that the console throws a syntax error regarding the variable `native`
- Visit http://jsfiddle.net/WyRs3/, which renames the variable to `nativePrototype`, and notice that no syntax error is thrown.