DevTools: fix changing of cursor while dragging

We used to change cursor upon drag on body only, but this now doesn't
have effect since we explicitly set cursor: default in .component-root.
So set the cursor explicitly both on the event target and body (the latter
is needed so that cursor change also has effect on the glass pane).

BUG=

Review URL: https://codereview.chromium.org/658723006

git-svn-id: svn://svn.chromium.org/blink/trunk@184356 bbb929c8-8fbe-4397-9dbb-9b2b20218538