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Re-enabled building of the browser_ui target on iOS (lost when
unit_tests was turned down). As with browser, removes non_android
sources and puts everything in non_mobile_sources.

See also https://codereview.chromium.org/785233002

BUG=432994

Review URL: https://codereview.chromium.org/801643006

Cr-Commit-Position: refs/heads/master@{#308448}