Copy a few selection related methods to WebFrame from WebView.

The old methods are temporarily left on WebView until Chrome switches.

Also remove WebView::isSelectionEditable since it's not called.

BUG=304341

Review URL: https://codereview.chromium.org/201773005

git-svn-id: svn://svn.chromium.org/blink/trunk@169968 bbb929c8-8fbe-4397-9dbb-9b2b20218538