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This change makes use of absl::variant inside of base::Value and
replaces the manually written tagged union.

Since adding an Abseil #include requires propagation of the
corresponding absl_include_config, several deps have been changed to
public_deps in order to avoid build errors.

TBR=battre

Bug: 646113
Change-Id: I781fb43aa0eb4caacbda7f745babd042755f199c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2332182
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799725}