DevTools: add status bar buttons to control transforms in Layers3DView

- add buttons to toggle between pan & rotate modes and perform reset;
- also add FrameViewer-like hotkeys ('x' & 'v')

Review URL: https://codereview.chromium.org/593413002

git-svn-id: svn://svn.chromium.org/blink/trunk@182623 bbb929c8-8fbe-4397-9dbb-9b2b20218538