Give a smaller viewport to webkit.org on devices

Give a smaller viewport to webkit.org on devices

Rubberstamped by Simon Fraser.

The CSS of WebKit.org layout the content on 615px + a right margin of
215px + a left margin of 20px. The total layout width is 850px.

The standard viewport size on mobile devices is 980px. This leaves a gap
on the right side of the content when opening the size on iPhone/iPad.

This patch changes the layout width to be 860 to display more content on devices
while keeping a little margin.

* header.inc:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@153853 268f45cc-cd09-0410-ab3c-d52691b4dbfc