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Currently interactive UI tests typically call `ClickOnView` to transfer
focus. On Mac, the behaviour of a click changes when an application
loses focus, and re-activating the application in a test is not
reliable. The test application shouldn't be losing focus in
interactive_ui_tests, but tracing suggests this is ocurring anyway.

This CL introduces ui_test_utils::FocusView which is a more robust way
to transfer focus in a test.

OmniboxViewTest.BeginningShownAfterBlur is changed to use
ui_test_utils::FocusView and re-enabled.

BUG=406012

Review URL: https://codereview.chromium.org/534993002

Cr-Commit-Position: refs/heads/master@{#293516}