Remove some defensive checks in StyleBuilderCustom

This patch removes some of the defensive checks in StyleBuilderCustom,
which quit the value applicators if the given CSSValue is not of the
expected type. These defensive checks make little sense and removing
them means that any parser bugs can be caught by the assertions, in the
toCSSFooValue functions or the CSSPrimitiveValue casts.

For the checks removed here, I've manually checked that the parser only
gives us appropriate CSSValues.

BUG=374530

Review URL: https://codereview.chromium.org/286203013

git-svn-id: svn://svn.chromium.org/blink/trunk@174326 bbb929c8-8fbe-4397-9dbb-9b2b20218538