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Adds a public/platform.h that is required to be implemented by
embedders to provide certain features.

Page-level allocation goes through this interface. This is used to
allow e.g.  Blink to provide their own OOM callbacks and page tags.

Provide a basic implementation of the PageAllocator using base
primitives that can be used for testing.

Bug: 1056170
Change-Id: Ib25269d6f8ed793d2b111dd750cef9d4a6d772a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083022Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747208}