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In addition to adding an application-level open URL utility, this CL
also converts the WebState visibility check to a unittest.

Bug: 987646
Change-Id: Ia2c99573dcabf64db0b06c094c95d5aa6d44c3df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1900690
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#723491}