If the subject string is a string, it's pointer-equal to the search
string, and position <= 0, then we can simply return 0 and skip
all other logic in String.p.indexOf.

Further context at:
https://twitter.com/hashseed/status/893539117367271425

Bug: 
Change-Id: I93ce724f0ade6332599ba395fe8c662a28f05ade
Reviewed-on: https://chromium-review.googlesource.com/602214Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47183}