Only the CompilationCache doesn't actually specialize the table to the point where it knows how to hash stored keys. This moves the virtual HashForObject down from HashTableKey to CompilationCacheKey, and moves previous virtual implementations to the respective shapes.

Bug: v8:6474
Change-Id: I0a1ae26a224d602d16692d2b09c96a2ab193f07f
Reviewed-on: https://chromium-review.googlesource.com/529110Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45842}