Use IDL default argument value for Range.collapse()'s toStart argument

Replace the existing [Default=Undefined] extended attribute, which since
undefined coerces to false means the same thing, with the actual default
value that's in the specification.

BUG=258153

Review URL: https://codereview.chromium.org/356103003

git-svn-id: svn://svn.chromium.org/blink/trunk@177235 bbb929c8-8fbe-4397-9dbb-9b2b20218538