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ComboboxModelObserver already exists that provides this functionality,
and ComboboxModel implementors already presume the existence of
ComboboxModelObserver via ComboboxModel::[Add|Remove]Observer.

Additionally, consumers of Combobox really shouldn't manually
call Combobox::ModelChanged(). This should be handled on the model
and the model itself needs to signal when changes occur.

This change as a result has the Combobox start listening to model
changes a la ComboboxModelObserver. This is a no-op for read-only
models as they get the no-op [Add|Remove]Observer default
implementation.

Dependent components have also been updated to account for this change
in behavior.

BUG=946299

Change-Id: Ie71949685dfa8015bad6fdb973512b7efb38c7a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610229Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659676}