Remove obsolete target-densitydpi documentation

Bug: 7999930

In general WebView.java is not a good place for HTML/CSS reference
guide, and more specifically target-densitydpi is an obsolete CSS
feature that is being phased out in modern browsers.
http://petelepage.com/blog/2013/02/viewport-target-densitydpi-support-is-being-deprecated/

Change-Id: Idd5cf09bb44e9f5f13f57e5f1c4b5c8ae1fc069b