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This is used by the Chrome on iOS today extension to get information
about the nearby devices and display them when user has enabled the
physical web integration.

Introduce a new target //ios:all to list all targets build on iOS
and add a dependency on it from //:gn_all. This will avoid changing
root BUILD.gn every time an iOS target is added upstream.

BUG=615799

Review-Url: https://codereview.chromium.org/2023833002
Cr-Commit-Position: refs/heads/master@{#397098}