[Extension Toolbar] Refactor and finish pop out logic for actions

[Extension Toolbar] Refactor and finish pop out logic for actions

* Pull out the logic to "pop out" actions when they want to act
  into a separate inner class of ToolbarActionsBar.
* Make unit tests for ToolbarActionsBar also test the overflow
  container.
* Turn off "popping out" actions by default.

UI has decided that, for the time being, we want to hold off
on popping out actions. Since this is still in heavy flux,
keep it in the code base (and tested) to prevent code rot.

BUG=417441

Review URL: https://codereview.chromium.org/766263003

Cr-Commit-Position: refs/heads/master@{#308638}