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This CL extends the LoadParams struct to allow loading a library from
an existing file descriptor and/or a reserved memory mapping as well.

This will be used later to implement a wrapper for android_dlopen_ext()
that support ANDROID_DLEXT_{USE_LIBRARY_FD, USE_LIBRARY_FD_OFFSET,
RESERVED_ADDRESS, RESERVED_ADDRESS_HINT}.

+ Provide new tests to verify that the feature works properly.
+ Fix FileDescriptor move assignment operator for 64-bit unit tests.

BUG=936001
R=pasko@chromium.org, agrieve@chromium.org, cjgrant@chromium.org

Change-Id: I1ed174dfa268f780c5397f6e76c659f8fa57c935
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1564111
Commit-Queue: David Turner <digit@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658519}