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This is a reland of cf629e19

Added import of constants.h file in unified_consent_view_controller.h so that
internal egtests can continue access the constants. After this CL lands, will
change the egtest header in internal code base and remove the constants.h
header import here.

Original change's description:
> Extract unified consent constants target.
>
> Preparing for SigninEarlGreyUI and SigninEarlGreyUtils EG2 migrations.
>
> Bug: 922813, 1015907
> Change-Id: I47e4678e40f1820b22938463c5a791c04a662f3c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884286
> Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
> Reviewed-by: Rohit Rao <rohitrao@chromium.org>
> Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#710479}

Bug: 922813, 1015907
Change-Id: I4584bfdda1a4854e17aac174ba1906b9e80adf09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1888601
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710614}