[Banner] apply constraints to trailingButton even it is hidden. (#7652)
[Banner] apply constraints to trailingButton even it is hidden. (#7652)

TrailingButton shows missing position constraints because no constraint was active when it is hidden.

| Before | After |
| --- | --- |
| <img width="1235" alt="Screen Shot 2019-06-19 at 3 49 18 PM" src="https://user-images.githubusercontent.com/8836258/59796048-a3b2a400-92aa-11e9-8d8b-085adbea481f.png"> | <img width="1301" alt="Screen Shot 2019-06-19 at 3 50 32 PM" src="https://user-images.githubusercontent.com/8836258/59796054-a7462b00-92aa-11e9-8e87-713603ec0ed3.png"> |


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