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This CL adds the container coordinator and coordinator factory for the
kWebContentArea modality.

Bug: 941745
Change-Id: Id539be9a3776ed80fc293578cd33394896e71117
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610095Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664922}