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At present, a change in the default zoom level does not immediately
update the zoom icon in the location bar. E.g. if we change the zoom
level of chrome://settings to 90% (causing the zoom icon to display a
'-' symbol), then change the default zoom level to be 75%, the icon
should switch to show '+'.

This CL adds an event subscription to the ZoomEventManager in order
to receive notifications about default zoom level changes.

BUG=451349

Review URL: https://codereview.chromium.org/897513003

Cr-Commit-Position: refs/heads/master@{#314568}