Refactor fixed layout mode

Previously, the desktop browser would use FrameView's rect
as the layout size while the mobile browser would enable
"fixed layout mode" which then used a layoutSize variable
that it could set instead.

This CL removes "fixed layout mode" as a FrameView concept
and makes the layout size always an explicitly set property
of the main FrameView. Another way to think about it is that
we're now always in a fixed-layout, only we keep
the layout size pegged to the window size (if the
--enable-viewport flag is set, the layout size is set from
@viewport/viewport meta).

Note: WebViewImpl still has a setFixedLayoutSize method 
that's used by Android WebView.

BUG=285397

Review URL: https://codereview.chromium.org/23819019

git-svn-id: svn://svn.chromium.org/blink/trunk@159039 bbb929c8-8fbe-4397-9dbb-9b2b20218538