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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}