Add BYTES_PER_ELEMENT property to constructors of typed arrays.

ES6 does not prescribe it, but both Blink and Firefox have it.

Also does a small rename of parameter in test.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/19562002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15712 ce2b1a6d-e550-0410-aec6-3dcde31c8c00