Remove explicit bounds check from CSSValueList::item

Currently CSSValueList::item does a bounds check and will return null if
the given index is out of range. This patch makes the interface slightly
more sane by removing the check, which is currently otherwise skipped by
calling itemWithoutBoundsCheck. Note that the vector access will perform
a bounds check (with release assert) anyway. The only caller where the
bounds check behaviour is useful is for the IDL binding so a separate
function itemWithBoundsCheck is created for this (already tested by
value-list-out-of-bounds-crash.html).

Review URL: https://codereview.chromium.org/333163004

git-svn-id: svn://svn.chromium.org/blink/trunk@177064 bbb929c8-8fbe-4397-9dbb-9b2b20218538