DevTools: Fix Ctrl+Shift shortcut handling

Drive-by: be more lenient when detecting printable key press combinations.

R=vsevik@chromium.org, vsevik
NOTRY=true

Review URL: https://codereview.chromium.org/244913002

git-svn-id: svn://svn.chromium.org/blink/trunk@172024 bbb929c8-8fbe-4397-9dbb-9b2b20218538