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This is a reland of 5906c258

This CL adds .def files for release builds (and changes the
generator script to generate them at roll time).

The issue was that release component builds (is_debug=false) were
trying to link symbols that were not available in release mode
.obj files.

Original change's description:
> Abseil component build (default symbols visibility).
>
> This CL introduces an Abseil component on which all Chromium Abseil
> clients will have to depend on.
>
> Abseil doesn't export symbols explicitly, so for the moment Chromium
> exports all of them when is_component_build=true. On Android, Linux and
> Mac -fvisibility=default is used while on Windows a .def file is
> generated at Abseil roll time.
>
> Bug: 1046390
> Change-Id: I1697eb9e0179cee01badcf1b687acd94160e5f6d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2174434
> Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#781225}

Bug: 1046390
Change-Id: Ieb48ec9eff6d08a0770c4b22521536bb29a76121
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2264062
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782701}