Fix mat-chip avatar image stretch issue

[JamesJansson]

Fix image stretch issue raised here:
#19822

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[crisbeto]

LGTM

[crisbeto]

It looks like the CLA hasn't been signed. We can't merge in the change until it is.

[JamesJansson]

It looks like the CLA hasn't been signed. We can't merge in the change until it is.

@crisbeto I have just signed the contributor agreement

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