DevTools: separate TextRange to its own file

This patch puts TextRange class into its own file. This will help
to remove TextEditorModel in close future.

TBR=vsevik

Review URL: https://codereview.chromium.org/22648003

git-svn-id: svn://svn.chromium.org/blink/trunk@155775 bbb929c8-8fbe-4397-9dbb-9b2b20218538