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This CL extracts CookieWaiter class from the GaiaScreenHandler and reuse
it in the EnrollmentScreenHandler.

Bug: b/167404108
Change-Id: Idaeca9cf93157b42819eac33a653eebe27def867
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429025Reviewed-by: Roman Aleksandrov <raleksandrov@google.com>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812160}