Refactor SVGPreserveAspectRatio::parseInternal()

This cl removes "goto" and replaces it with local variables, and fixes poor coding style.

R=schenney
BUG=

Review URL: https://codereview.chromium.org/144463009

git-svn-id: svn://svn.chromium.org/blink/trunk@166927 bbb929c8-8fbe-4397-9dbb-9b2b20218538