Use website preferred width as the fixed viewport.

This preferred width will be set between the default viewport width and
the max fixed viewport width.  The default viewport width serves as the
minimum fixed viewport width, so it's lowered to 980.

This change will make the shirt.woot.com/blog site works better, since it's using
1080 as the preferred width, so user needs not to scroll in overview mode.

issue: 3003933
Change-Id: Ie3eed44a54d277f5bc5f63cd394d463012a80d80