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This is a reland of aa7438aa

Unmodified from the original CL, as breakage was from a precursor
that has since been fixed and relanded.

Original change's description:
> Convert some mojo_base typemaps to GN targets
>
> Uses new GN-based typemap definitions to replace some typemap files in
> mojo/public/cpp/base.
>
> Bug: 1059389
> Change-Id: If1389c9ceec8ae90df753895af0abd9f72c704a0
> Tbr: dtapuska@chromium.org
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2090723
> Reviewed-by: Ken Rockot <rockot@google.com>
> Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
> Reviewed-by: Tom Sepez <tsepez@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Commit-Queue: Ken Rockot <rockot@google.com>
> Cr-Commit-Position: refs/heads/master@{#748056}

Bug: 1059389
Change-Id: I5bbe5afff09493b3394f8922c3fe190e731305b7
Tbr: dcheng@chromium.org
Tbr: tsepez@chromium.org
Tbr: oksamyt@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095929Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#748569}