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Page actions in the location bar have been somewhat loosely tested as a side-
effect of the page actions api testing, but (near as I can tell), we never
explicity test the ui for page actions.

This is a problem because the extensions tests shouldn't care about the UI, and
soon won't, since where the page actions are in the UI may change (i.e.,
location bar vs toolbar).

Fix this by adding a few basic UI tests for page actions in the location bar.

BUG=408261

Review URL: https://codereview.chromium.org/512693003

Cr-Commit-Position: refs/heads/master@{#292957}