Bug
1792146: Make replace_malloc_usable_size succeed with pointers within allocation pages. r=glandium
This changes replace_malloc_usable_size to allow measuring the size of
ptrs that appear anywhere within an allocation page, not just ptrs to the
start of the allocation pages.
This also modifies a gtest to explicitly check the usable size of some
modified pointers, not just the usable size of the pages where those
pointers are located.
Differential Revision: https://phabricator.services.mozilla.com/
D170963