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- Added new files for constants.
- Added constants target in BUILD.gn for new files.
- Added/Modified headers of where the constants are used.

This is preparing for SigninEarlGreyUI and SigninEarlGreyUtils EG2
migrations.

Bug: 922813, 1015907
Change-Id: I41fc3223ebef4ba4e52e7f60e670eb6fdb0b42b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1885225
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@{#710496}