In strong mode, whenever either operand to an addition is a string, both
must be strings, so we can just use a simple string map check instead of
the STRING_ADD_LEFT / STRING_ADD_RIGHT machinery, which tries to do sloppy
and strict mode conversions before giving up.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1287203002

Cr-Commit-Position: refs/heads/master@{#30146}