[FlexibleHeader] Add missing self-> within a block.
[FlexibleHeader] Add missing self-> within a block.

Resolves the following error:
"Block implicitly retains 'self'; explicitly mention 'self' to indicate this is intended behavior"

PiperOrigin-RevId: 293801599

material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios