Update according to new spec change at
https://github.com/tc39/ecma262/pull/856

- Call ToNumber only once in BUILTIN
- Remove unused FillNumberSlowPath
- FillImpl assumes obj_value->IsNumber() is true
- Update test

Bug:v8:5929,chromium:702902

Change-Id: Ic83e6754d043582955b81c76e68f95e1c6b7e901
Reviewed-on: https://chromium-review.googlesource.com/465646Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44373}