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Where the old UnalignedSharedMemory type is created from a legacy
SharedMemoryHandle, this new type is created from a new shared memory
API instance of UnsafeSharedMemoryRegion. This changes both the
ownership semantics and mapping details from UnalignedSharedMemory: the
new UnalignedWritableMapping does not own the region backing the
shared memory, and only owns the mapping itself. Because of this, the
region is mapped upon construction rather than with a later MapAt
call.

These changes do not seem to affect behavior in a practical manner.

This change also removes the read-only bit. This is no longer
meaningful, as only the mapping is owned; in addition the original use
of this variable was not correct, as explained in detail in
https://goo.gl/HmBYy6. (TL;DR: on some platforms, being read-only is a
characteristic of the underlying OS region, and cannot be set in some
processes without affecting the protection in all processes. As this
behavior is incompatible with usage, this means that sometimes shared
memory which has been declared read-only is not, in fact, read-only, and
could be exploited by rouge processes).

Finally, a the new UnalignedWritableMapping contains a shim
constructor which takes a legacy SharedMemoryHandle. The ownership
semantics are the same a the region-created version, that is, the handle
is *not* owned by the UnalignedWritableMapping instance.

This shim allows us to convert all users of this class one-by-one, and
confirm that the new ownership semantics are correctly handled. After
this is done, the BitstreamBuffer class, which is at the heart of the
IPCs involved, can be converted to the new API without needing to change
ownership semantics at the same time.

See the bug for an overview of the entire process.

Bug: 849207
Change-Id: Ibe4bbb48f7bea3fb31728bf98133ede0228b9801
Reviewed-on: https://chromium-review.googlesource.com/1116703Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Alexandr Ilin <alexilin@chromium.org>
Commit-Queue: Matthew Cary <mattcary@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570930}