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The rationale is so tests can wait for this event before testing
functionality. Otherwise, tests will always fail because the data
takes time to load so the requisite elements haven't been created yet.

Change-Id: I5561273c9d17e010083615a6de513cd5c1f32184
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2636011
Commit-Queue: Kenton Lam <kentonlam@google.com>
Reviewed-by: Keith Lee <keithlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845068}