Allow fround to take values without dots for globals (the spec allows this
subtly).

Drop over-restrictive assert preventing floating point globals from working.

BUG=v8:4203
R=jpp@chromium.org,aseemgarg@chromium.org

Review-Url: https://codereview.chromium.org/2397823003
Cr-Commit-Position: refs/heads/master@{#40013}