Don't propagate keydown events when dialog is open

We shouldn't propagate keydown events so that it can
behave modally.

Bug: 883221
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I4aa5257b4bf6c417123d659ba911cb01641a4cc7
Reviewed-on: https://chromium-review.googlesource.com/1230493
Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Sang Woo Ko <sangwoo108@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593619}
3 files changed