This is necessary in order to support class literals boilerplates which will
contain descriptor arrays not attached to maps.

This CL introduces Heap::descriptor_array_map() which will be used only for
descriptor arrays templates created as a part of class literal boilerplate.

We don't use the new map for all descriptors for now because in order to handle
chicken-egg problem in deserializer which can be solved in a clean and readable
way only if introduce a DESCRIPTOR_ARRAY_TYPE instance type which we can't
currently do because the InstanceType enum is already "full".

Bug: v8:5799
Change-Id: I732d236b0dda2c436ab3ce3ac5967f6c66162df3
Reviewed-on: https://chromium-review.googlesource.com/758360
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49271}