Resubmit: Refactor external_install_ui

Resubmit: Refactor external_install_ui

Original CL: https://codereview.chromium.org/300853008/
Original Description:
Refactors ExternalInstallUI to have determinate lifetimes.
This is likely the first of multiple patches targeting this area of code; the
next one will concentrate on pulling the related logic out of ExtensionService.

----------------------------------------------------

The original patch contained a cycle in that when an external install warning
was clicked, it would accept or remove the extension, and then remove the error.
Since the manager listened for the extension to be loaded or unloaded, this
resulted in a double-removal.  Fix is straightforward.

We also didn't have tests for actually interacting with the dialog, so two have
been added.

TBR=yoz@chromium.org
BUG=378042
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=274325

[Devlin, 2014-06-02 16:49:01]

The CQ bit was checked by rdevlin.cronin@chromium.org

[commit-bot: I haz the power, 2014-06-02 16:49:20]

CQ is trying da patch. Follow status at

https://chromium-status.appspot.com/cq/rdevlin.cronin@chromium.org/309643007/...

[commit-bot: I haz the power, 2014-06-02 20:53:10]

Change committed as 274325

[Devlin, 2014-06-04 20:04:19]

A revert of this CL has been created in
https://codereview.chromium.org/313173003/ by rdevlin.cronin@chromium.org.

The reason for reverting is: Led to increased crashiness:
https://code.google.com/p/chromium/issues/detail?id=368809.