[ios] Splits history_ui_constants into a separate GN target.

This will allow EG2 tests to use these constants without pulling in
more of //ios/chrome/browser/ui.

BUG=None

Change-Id: Ifb5dbeaf841b598767a63521aea6e07bb5f9dc4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1618123
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661377}
3 files changed