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Adds an identifier for EGTests to each window. This will allow matching
specific windows in tests.

Bug: none
Change-Id: I9d04ee645ec1658181d5580a8f4c2060b6927940
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463565
Auto-Submit: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816167}