Remove convertToRootView and convertFromRootView from Widget.

These are identical to convert{To,From}ContainingWindow.

Review URL: https://codereview.chromium.org/590253002

git-svn-id: svn://svn.chromium.org/blink/trunk@182689 bbb929c8-8fbe-4397-9dbb-9b2b20218538