Add some protections against partitionAlloc metadata corruption.

I added:
- Simple double-free detection, a la tcmalloc.
- Robustness against partial pointer overwrites of freelist entries.
- Protect partition page headers with a simple / fast canary that is solid
against the important threat, which is where the attacker has not defeated
ASLR.

I pulled a few tricks to do so with little measurable performance change;
details in the bug.

BUG=249137

Review URL: https://codereview.chromium.org/28903002

git-svn-id: svn://svn.chromium.org/blink/trunk@160024 bbb929c8-8fbe-4397-9dbb-9b2b20218538