Use the new ToString_Inline function instead, which performs a quick
IsString check and calls the ToString builtin to handled conversion.

This reduces builtins code size by 3K.

BUG=v8:5737

Change-Id: I103e628b905aed9d74dd7b4c4a98c5b0a16fd476
Reviewed-on: https://chromium-review.googlesource.com/527133Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45768}