The Math.expm1() function can actually return -0, for example in the
case that -0 is passed to it.

Bug: chromium:880207
Change-Id: If3a7a3a1fb6a18075ba0d7816687dfd831ebe293
Reviewed-on: https://chromium-review.googlesource.com/1205072Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55657}