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This CL adds all handoff manager tests to EG2 target
and updates headers and adds eg2_test source_set in
build file. Moreover, new app interface for handoff
tests is being introduced.

Bug: 987646
Change-Id: I3410f778e9f9daece1767a4f0a73c168f6e9aef0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872209Reviewed-by: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Nazerke Kalidolda <nazerke@google.com>
Auto-Submit: Nazerke Kalidolda <nazerke@google.com>
Cr-Commit-Position: refs/heads/master@{#708953}