String wrappers (new String("foo")) are special objects: their string
characters are accessed like elements, and they also have an elements
backing store. This used to require a bunch of explicit checks like:

if (obj->IsJSValue() && JSValue::cast(obj)->value()->IsString()) {
  /* Handle string characters */
}
// Handle regular elements (for string wrappers and other objects)
obj->GetElementsAccessor()->Whatever(...);

This CL introduces new ElementsKinds for string wrapper objects (one for
fast elements, one for dictionary elements), which allow folding the
special-casing into new StringWrapperElementsAccessors.

No observable change in behavior is intended.

Review URL: https://codereview.chromium.org/1612323003

Cr-Commit-Position: refs/heads/master@{#33616}