Clean up parser and StyleBuilder handling of baseline-shift

This patch fixes a typo in parsing of baseline-shift, removes the
defensive code for applying the property, and adds an assertion to
SVGLength::fromCSSPrimitiveValue that we have a type we can handle
(note that the parsing of svg properties only allows the length types
listed there and not vw, etc.)

BUG=374530

Review URL: https://codereview.chromium.org/290793002

git-svn-id: svn://svn.chromium.org/blink/trunk@180895 bbb929c8-8fbe-4397-9dbb-9b2b20218538