This method used to be defined on Object and handled Strings and
JSObjects; but only the object hierarchy rooted at JSObject has
'elements', and Strings are handled slightly differently. Thus it
makes sense to split up into

 JSObject::GetOwnConstantElement
 String::GetCharAsString

This way, we can also separate future work on making JSObjects and
Strings never-serialized.

Bug: v8:7790
Change-Id: I8e0f142fbd9cbf8e8abe1e9a189bcd948c2f1fa8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2704080
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72911}