Revert of Resubmit: Refactor external_install_ui

Revert of Resubmit: Refactor external_install_ui (https://codereview.chromium.org/309643007/)

Reason for revert:
Led to increased crashiness: https://code.google.com/p/chromium/issues/detail?id=368809

Original issue's description:
> 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

TBR=yoz@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=378042
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=274890

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

Created Revert of Resubmit: Refactor external_install_ui

[commit-bot: I haz the power, 2014-06-04 20:05:30]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/rdevlin.cronin@chromium.org/313173003/1

[commit-bot: I haz the power, 2014-06-04 20:07:27]

Change committed as 274890