Temporarily disable assert in FrameView::updateWidgets().

This assert is causing PPAPI tests to flakily fail on the Linux Chromium
ChromeOS trybot.

BUG=430344
TBR=zeeshanq@chromium.org

Review URL: https://codereview.chromium.org/704673005

git-svn-id: svn://svn.chromium.org/blink/trunk@184883 bbb929c8-8fbe-4397-9dbb-9b2b20218538