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Before:
* The security icon on the toolbar did not have content description,
i.e, when the accessibility assistant did not speak when users focused
the security icon.
* The mediator uses static methods from these classes that make it hard
to test:
 - SecurityStatusIcon
 - SecurityStateModel
 - PageInfoController

After:
* The security icon has a content description. The description is the
same as the one used on custom tab toolbar.
* The static methods are invoked from the coordinator

Change:
* Add content description to security icon.
* Move the classes with static methods out of the mediator to make it
testable.

Bug: 1067659

Change-Id: Icd21f0578492e9b9b595cd06909049839c6eed85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2140956
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759875}