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The test creates a fake RELRO region in private memory and checks that
it is spawned into a sealed read-only shared memory region.

Since the normal way of using the ModernLinker involves a separate DSO
(=shared library), this would add more moving parts to base_unittests.
We prefer less advanced ways with GN, so another approach is chosen: all
the necessary code is linked with base_unittests.

Testing the way ModernLinker loads libraries would be difficult without
extra libraries pushed to the device, but RELRO region creation, sharing
and consumption can be tested using this approach.

Bug: 1154224
Change-Id: Ia78cce98f566084f6f799a7a35ab3cc45beb9346
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2571758Reviewed-by: Benoit L <lizeb@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833793}