This makes sure the {CodeSpaceMemoryModificationScope} for the common
allocation path is inside the {Heap} component. This will in turn enable
finer-grained control of the modification scope in the future.

R=hpayer@chromium.org
BUG=v8:6792

Change-Id: I6c3bc457bac641e79b2786cf78557b26aa2027e7
Reviewed-on: https://chromium-review.googlesource.com/779399
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49532}