Clean up handling of autosizing state changes.

All page-level state changes now go through updatePageInfo, which marks blocks
as needing layout if their multipliers might need updating.

This simplifies the flow and ensures the page updates correctly on orientation
changes and movements of the text scaling slider in accessibility settings.

BUG=353309,347384

Review URL: https://codereview.chromium.org/209353003

git-svn-id: svn://svn.chromium.org/blink/trunk@169997 bbb929c8-8fbe-4397-9dbb-9b2b20218538