Also drive-by cleanup the TestMemoryAllocatorScope class so that it
takes ownership of the old allocator while it holds onto it, and so
that the MemoryAllocator for testing is constructed inside the scope
rather than passed into it. This means users don't need to explicitly
call TearDown() and delete the allocator as the scope does it for them.

Change-Id: Id7da3c074618a376d2edfe3385bb185ba8287cea
Reviewed-on: https://chromium-review.googlesource.com/c/1392194
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59627}