DevTools: Fix DefaultFocusedViewStack handling for dialogs.

BUG=392793
R=apavlov, dgozman

Review URL: https://codereview.chromium.org/403493003

git-svn-id: svn://svn.chromium.org/blink/trunk@178372 bbb929c8-8fbe-4397-9dbb-9b2b20218538