Fixed add to tree button on mobile

Phil Hughesrequested to merge
new-file-mobile-dropdown-fix into master

What does this MR do?

Fixes an issue with the width of the add to tree button on mobile.

This was caused by acde2e30 adding 100% width for mobile.

Screenshots (if relevant)

Screen_Shot_2016-09-14_at_10.52.41

What are the relevant issue numbers?

Closes #22128 (closed)

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