Inline String::substring() to avoid unnecessary function call when the result is the same string.
Pull request: https://github.com/WebKit/WebKit/pull/3911
Committed 254074@main (cc6daf68c1ca): <https://commits.webkit.org/254074@main> Reviewed commits have been landed. Closing PR #3911 and removing active labels.
<rdar://problem/99459784>