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This CL adds TextBadgeView to iOS Showcase. This involves adding a
text_badge_view folder to showcase, creating a glue view controller,
and modifying the Showcase configuration file and BUILD.gn files as
necessary.

Since TextBadgeView is currently a stub implementation, Showcase
displays a blank white screen.

BUG=740133.

Review-Url: https://codereview.chromium.org/2967113003
Cr-Commit-Position: refs/heads/master@{#485302}