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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197182

[Chris Evans, 2015-06-16 07:44:13]

cevans@chromium.org changed reviewers:
+ haraken@chromium.org

[haraken, 2015-06-16 09:35:51]

LGTM

[Chris Evans, 2015-06-16 17:16:00]

The CQ bit was checked by cevans@chromium.org

[commit-bot: I haz the power, 2015-06-16 17:16:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1185303003/1

[commit-bot: I haz the power, 2015-06-16 17:20:01]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197182