Makes all but one data member private instead of protected and replaces
all Space::heap_ accesses with Space::heap().

Also moves Executability down from Space into PagedSpace and remove all
references in SemiSpace since it's always initialized with
NOT_EXECUTABLE.

Bug: v8:7754
Change-Id: Ic03ce35a5f970b3c1e25b32da53e4c9717b2ee1e
Reviewed-on: https://chromium-review.googlesource.com/1055510Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53146}