[Merge to M55] Fix regression in handling toggle mirror mode

Toggle mirror mode shouldn't be dependent on the dev-shortcuts flag.

TBR=jamescook@chromium.org
BUG=650394
TEST=Ctrl+F4 should toggle mirror mode successfully.

Review-Url: https://codereview.chromium.org/2417773002
Cr-Commit-Position: refs/heads/master@{#425189}
(cherry picked from commit d8b394e863e69c0b99338e826a05fae76f22851a)

Review URL: https://codereview.chromium.org/2423673002 .

Cr-Commit-Position: refs/branch-heads/2883@{#129}
Cr-Branched-From: 614d31daee2f61b0180df403a8ad43f20b9f6dd7-refs/heads/master@{#423768}
1 file changed