Improve seeding of AddressSpaceRandomization randomness.
Improve seeding of AddressSpaceRandomization randomness.

- Mix in a bit of time as well as pid. (On Linux, pids can be identical
between processes because of namespacing.)
- We use raw calls to fetch the time, to keep PartitionAlloc standalone and
not depend on a platform callback. Furthermore, we're deep inside an allocator
at this point and the platform callbacks are not simple and might re-enter a
memory allocator which is a no-no. For consistency, bring the process id
related calls inline.
- Also add a TODO to improve the randomization range on Win8.1+.

BUG=499465

Review URL: https://codereview.chromium.org/1185303003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@197182 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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