DevTools: shortcut to close active editor tab

This patch assigns Alt-W shortcut to close active editor tab.

BUG=338324

Review URL: https://codereview.chromium.org/148233003

git-svn-id: svn://svn.chromium.org/blink/trunk@165872 bbb929c8-8fbe-4397-9dbb-9b2b20218538