Mac: Prevent a held-down Enter key from closing the Open File dialog

Bug: 637098
Change-Id: I18ddab35db1ff3155b847576880de4df4e1290ba
Reviewed-on: https://chromium-review.googlesource.com/875522
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531035}