As a followup of [JDK-8302590](https://bugs.openjdk.org/browse/JDK-8302590), this issue covers the analogous case for a search of a string rather than a character.
------------- Commit messages: - 8303648: Add String.indexOf(String str, int beginIndex, int endIndex) Changes: https://git.openjdk.org/jdk/pull/12903/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12903&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8303648 Stats: 251 lines in 2 files changed: 236 ins; 5 del; 10 mod Patch: https://git.openjdk.org/jdk/pull/12903.diff Fetch: git fetch https://git.openjdk.org/jdk pull/12903/head:pull/12903 PR: https://git.openjdk.org/jdk/pull/12903
