Remove obsolete forward declarations from SVGDocument.

Additionally, add explicit keyword to constructor that takes one argument.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/24069002

git-svn-id: svn://svn.chromium.org/blink/trunk@157442 bbb929c8-8fbe-4397-9dbb-9b2b20218538