Don't squash iframes, frames or plugins.

This won't work in some corner cases, due to non-squashing-aware code in 
FrameView.

BUG=

Review URL: https://codereview.chromium.org/356453002

git-svn-id: svn://svn.chromium.org/blink/trunk@176864 bbb929c8-8fbe-4397-9dbb-9b2b20218538