DevTools: pinch zoom around current cursor location while emulating touch.

R=vsevik@chromium.org
TBR=vsevik

Review URL: https://codereview.chromium.org/54993007

git-svn-id: svn://svn.chromium.org/blink/trunk@161157 bbb929c8-8fbe-4397-9dbb-9b2b20218538