Fixed positioned element with relative percentage positioned parent is wrongly placed

The usage of Length::value() is causing a bug as 25% was translated to 25px,
which is wrong.
Use valueForLength() instead in order to get the accurate value.

Modified current test to add some examples using percentages.

BUG=31286,425991
TEST=fast/css/css-properties-position-relative-as-parent-fixed.html

Review URL: https://codereview.chromium.org/720003002

git-svn-id: svn://svn.chromium.org/blink/trunk@185274 bbb929c8-8fbe-4397-9dbb-9b2b20218538