This adds a new %ClearMegamorphicStubCache() runtime function, which can
be used to easily wipe the megamorphic stub cache. This is useful when
investigating performance issues that are related to high contention on
the megamorphic stub cache.

Change-Id: I7abb496bf1dd658298a90065241dd317f2fb6624
Reviewed-on: https://chromium-review.googlesource.com/c/1459637
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59456}