Use C++11 range-based loop in core/editing

Improves code readabilty

BUG=none
TEST=no layout test failures

Review URL: https://codereview.chromium.org/659933004

git-svn-id: svn://svn.chromium.org/blink/trunk@184626 bbb929c8-8fbe-4397-9dbb-9b2b20218538