[ios] Imports Foundation.h instead of UIKit.h in popup_menu_constants.h.

This file only uses NSString, so it does not need all of UIKit.h.
Switching to Foundation.h makes it easier to compile this file into the
EG2 test process.

BUG=None

Change-Id: Id76cf9ed46263a996a5d696cc9bdf5e04cf38eba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619830
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661350}
1 file changed