Replace conversions of relative indices in builtin-string-gen.cc with
ConvertToRelativeIndex which is defined in code-stub-assembler.cc.

Change-Id: I790d58d296ccb482d3f96a62ec64227b1c2ce2c0
Reviewed-on: https://chromium-review.googlesource.com/883701Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50880}