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As another step towards a platform-agnostic test suite for the password
management UI, this CL refactors ManagePasswordsIconView in a few ways:

1. It introduces a new 'ManagePasswordsIcon' abstract base class
   to provide a platform-agnostic interface to the Omnibox icon.

2. The ManagePasswordsBubbleUIController is now responsible for
   determining the Omnibox's icon's state, and also responsible
   for triggering automatic display of the bubble UI. The actual
   functionality is still sitting on the icon for the moment, but
   will be moved out to a platform-agnostic layer in a future CL.

3. Most importantly, some basic unit tests verify the functionality
   added above.

BUG=356678

Review URL: https://codereview.chromium.org/250353003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266520 0039d316-1c4b-4281-b951-d872f2087c98