We use the naming convention <Constructor>Prototype<Method> for builtins
that implement methods on a certain builtin constructors prototype. Fix
the collection builtins (Map and Set) to match this naming convention.

Bug: v8:5049
Change-Id: I8ced50c2ac9ebc8f4390bcbbc6aec426a0026813
Reviewed-on: https://chromium-review.googlesource.com/707318Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48403}