Add RenderWidget::UpdateSuspendScope

WidgetHierarchyUpdatesSuspensionScope exposed a bunch of static methods
that no one should call and are only used by RenderWidget. Instead
fully encapsulate the logic into RenderWidget.

Review URL: https://codereview.chromium.org/25752004

git-svn-id: svn://svn.chromium.org/blink/trunk@158995 bbb929c8-8fbe-4397-9dbb-9b2b20218538