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In preparation for moving to multiple HostZoomMaps (one-per
StoragePartition), it is helpful to simplify ZoomController by removing
the prefs machinery for DefaultZoomLevel, and instead rely on getting
this value from the HostZoomMap.

The calls to ZoomController::UpdateState() that used to be triggered
directly by the prefs changes still occur, though by a roundtrip from
PrefsTabHelper via IPC to RenderViewImpl and then by IPC back to
the HostZoomMap, which in turn notifies its observers. This only happens
in cases where the change in DefaultZoomLevel actually causes the
RenderViewImpl to change its zoom, so it is a subtle change in behaviour
from before.

BUG=335317

Review URL: https://codereview.chromium.org/508263002

Cr-Commit-Position: refs/heads/master@{#292920}