Introduce a new macro TO_STRING that maps to %_ToString and use that
instead of calling into any of the ToString/NonStringToString JavaScript
builtins. Also remove the TO_STRING_INLINE macro, which is basically
obsolete with %_ToString. We still have a few uses of ToString left (via
the utils export mechanism), where we need to investigate whether we
will tank badly if we replace them with TO_STRING as well.

CQ_INCLUDE_TRYBOTS=tryserver.v8:v8_linux_layout_dbg,v8_linux_nosnap_dbg
R=yangguo@chromium.org
BUG=v8:4307
LOG=n

Review URL: https://codereview.chromium.org/1323543002

Cr-Commit-Position: refs/heads/master@{#30895}