Add support for SmartClip

SmartClip is a feature used on Samsung devices via the Android WebView. It
finds nodes and text related to a given clip retangle and creates an encoded
string which is interpreted by Samsung's SmartClip module.

This patch is based on https://codereview.chromium.org/112423003/ by Juhui Lee <juhui24.lee@samsung.com>.

Review URL: https://codereview.chromium.org/118803002

git-svn-id: svn://svn.chromium.org/blink/trunk@164221 bbb929c8-8fbe-4397-9dbb-9b2b20218538