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This is a reland of 888c9108

Original change's description:
> Delete HeadlessBrowserContext::Observer::OnChildContentsCreated.
> 
> This isn't used.
> 
> Change-Id: Id61efee5ce35617c6ab65c6c19a059b36323176f
> Reviewed-on: https://chromium-review.googlesource.com/1065076
> Reviewed-by: Alex Clarke <alexclarke@chromium.org>
> Commit-Queue: Johannes Henkel <johannes@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#559821}

Change-Id: I52e6f2361da7dd900eef24dea4aefd57fa8dda9a
Reviewed-on: https://chromium-review.googlesource.com/1073619Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563046}