This CL introduces SloppyArgumentsElements to encapsulate all the constants
for SLOW_ and FAST_SLOPPY_ARGUMENTS_KINDS. This will serve as a better
documentation and reduces the use of undocumented constants.

Change-Id: I7a5b4e79f02573161d8a83aaf6f69fc490883aa5
Reviewed-on: https://chromium-review.googlesource.com/467666Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44433}