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Add sampling shims for PartitionAlloc allocations to the GWP-ASan
client. One of PartitionAlloc's core security guaranteees is that
allocations with different types are always segregated in memory so that
use-after-frees can only be exploited within a given partition.

For now I have not yet taught the GuardedPageAllocator about types,
instead I only allow allocating every page a single time so that no two
allocations overlap to ensure this property. This is sufficient for a
prototype to test in-the-wild but will need to be fixed before enabling
widely.

Crash handler support and wiring into chrome/ will follow.

Bug: 956824
Change-Id: If98f6ca2b6aa9d8ffc1d5300e319d0c38c71458b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1597192
Auto-Submit: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657097}