Make Array.prototype.copyWithin generic.

According to http://www.ecma-international.org/ecma-262/6.0/#sec-array.prototype.copywithin Array.prorotype.copyWithin is a generic method. The extern definition is redefining the template T but is missing the @this annotation.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=244697432

Author: jDramaix

externs/es6.js

@@ -1359,6 +1359,7 @@ Array.prototype.fill = function(value, opt_begin, opt_end) {};
  * @param {number} start
  * @param {number=} opt_end
  * @see http://www.ecma-international.org/ecma-262/6.0/#sec-array.prototype.copywithin
+ * @this {!IArrayLike<T>|string}
  * @template T
  * @return {!Array<T>}
  */