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This is a reland of cdd8661f

Reason for reland:
The failing test PlatformSharedMemoryRegionTest.MapAtWithOverflowTest
was fixed.

Original change's description:
> base: Introduce the new Shared Memory API
>
> Design doc (public): https://goo.gl/HmBYy6
>
> This patch proposes the new API for the Shared Memory based on two
> concepts, a shared memory region and a shared memory mapping. The
> region controls the platform-specific handle and mediates sharing and
> access control. The mapping is just a reference to the memory bytes
> which knows how to unmap itself. Ownership is per-instance: neither a
> region nor a mapping may be copied, and resources are freed when then
> instance dies. This is accomplished through move-only semantics.
>
> The goal of the new API is to provide a correct cross-platform
> implementation that makes the sharing mode explicit via the C++ type
> system.
>
> Thus, this CL introduces two classes for shared memory mappings:
> - WritableSharedMemoryMapping owns and provides an access to writable
> shared memory
> - ReadOnlySharedMemoryMapping for read-only shared memory
>
> and three classes representing shared memory regions:
> - UnsafeSharedMemoryRegion allows to create
> WritableSharedMemoryMapping instances and cannot be used for making
> read-only mappings
> - ReadOnlySharedMemoryRegion, on the contrary, can produce only
> ReadOnlySharedMemoryMapping instances and it also creates one writable
> mapping in the caller process address space
> - WritableSharedMemoryRegion allows to create writable
> mappings and can be eventually converted to ReadOnlySharedMemoryRegion.
> The important limitation in comparison with UnsafeSharedMemoryRegion
> is that the WritableSharedMemoryRegion cannot be duplicated so in
> order to pass the writable region to another process, the ownership of
> the region must be moved/transferred to this process.
>
> Bug: 795291
> Change-Id: I7d1d0a24932145cc0f449b01ceb8cafef9b7e407
> Reviewed-on: https://chromium-review.googlesource.com/893458
> Commit-Queue: Alexandr Ilin <alexilin@chromium.org>
> Reviewed-by: Matthew Cary <mattcary@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Reviewed-by: Erik Chen <erikchen@chromium.org>
> Reviewed-by: Robert Sesek <rsesek@chromium.org>
> Reviewed-by: Ken Rockot <rockot@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#545521}

TBR=dcheng@chromium.org,erikchen@chromium.org,rsesek@chromium.org,rockot@chromium.org

Bug: 795291
Change-Id: Ibba9aac8e7ca5bb6c55a2e0ddf62178fdc8fae20
Reviewed-on: https://chromium-review.googlesource.com/979653
Commit-Queue: Alexandr Ilin <alexilin@chromium.org>
Reviewed-by: Alexandr Ilin <alexilin@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Matthew Cary <mattcary@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546053}