SVGLengthList should be cleared when specified empty string.

TEST=svg/dom/SVGLengthList-setEmptyString.html
BUG=362685

Review URL: https://codereview.chromium.org/238573004

git-svn-id: svn://svn.chromium.org/blink/trunk@171546 bbb929c8-8fbe-4397-9dbb-9b2b20218538