Remove non-sensical code to force body/documentElement layout in FrameView::layout

FrameView::layout() should only be about updating the layout of renderers which
need a layout update. It should not need to force renderers to layout.  If it's
doing that, then some other code has failed to correctly invalidate the layout
of those renderers.

This used to be for FrameFlattening (which is still part of seamless), but has morphed over the years.  I'm not sure it was ever really needed.  See:
http://trac.webkit.org/browser/trunk/WebCore/page/FrameView.cpp?annotate=blame&rev=73116#L501

And:
http://trac.webkit.org/changeset/73117

BUG=261689
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/19761008

git-svn-id: svn://svn.chromium.org/blink/trunk@154544 bbb929c8-8fbe-4397-9dbb-9b2b20218538