MessagePopupCollection: Include <memory> for std::unique_ptr

This fixes the libstdc++ build after commit d540c125c1 ("New implementation
of MessagePopupCollection").

Bug: 819294
Change-Id: Ibf45feee935b25d48564f84326b95f33529ab879
Reviewed-on: https://chromium-review.googlesource.com/1172784
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Raphael Kubo da Costa (CET) <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#582622}
1 file changed