Setup LayoutState for a subtree layout.

When doing a layout from a subtree (i.e. SVG) we would be missing the margins
for the body element when calculating the rect for repaint. This CL makes sure
the LayoutState is setup correctly for a subtree layout.

BUG=320139

Review URL: https://codereview.chromium.org/237003003

git-svn-id: svn://svn.chromium.org/blink/trunk@171572 bbb929c8-8fbe-4397-9dbb-9b2b20218538