Remove Widget::clipRectChanged notification

There is no reason for this notification to exist. The only client of this
notification is WebPluginContainerImpl, and that class just uses it to send a
general notification that will be sent during widgetPositionsUpdated anyway.

This CL is an incremental step towards avoiding computing the widget's clip
rect during layout because the clip rect depends on compositing information and
therefor creates a chicken/egg situation.

R=eseidel@chromium.org
BUG=339898

Review URL: https://codereview.chromium.org/154083009

git-svn-id: svn://svn.chromium.org/blink/trunk@166993 bbb929c8-8fbe-4397-9dbb-9b2b20218538