Move testRunner.textSurroundingNode to internals

The idea is as same as https://codereview.chromium.org/176953009/.
textSurroundingNode accesses a Node while gin::Wrappable
should not do it.

BUG=331301

Review URL: https://codereview.chromium.org/186533002

git-svn-id: svn://svn.chromium.org/blink/trunk@168711 bbb929c8-8fbe-4397-9dbb-9b2b20218538