The omnibox PRD indicates that the app button should cancel omnibox edit mode.  I clarified with gordonbrander@ why this is the case, when the button does not cancel voice input.

Gordon considered this, and concluded that the app button should indeed exit voice input mode, returning to the previous mode, just like full-screen.

If voice input was entered from omnibox editing mode, then the user should be returned to omnibox editing mode (not to content viewing mode).

Support for exiting omnibox is tracked in a separate bug.