Call convertToLength in CSSToStyleMap instead of having it inlined

Trivial cleanup; reduce copy-paste in CSSToStyleMap. Note that the
case where the primitive value isn't an appropriate type shouldn't
occur since we generally assume the parser only allows valid types
through so we change it to an assertion here.

BUG=322365

Review URL: https://codereview.chromium.org/98193002

git-svn-id: svn://svn.chromium.org/blink/trunk@162958 bbb929c8-8fbe-4397-9dbb-9b2b20218538