Add CSS display-mode experimental support for picture-in-picture

This CL adds experimental support to the CSS display-mode media feature
for picture-in-picture.

Spec PR: w3c/csswg-drafts#9920
Intent to prototype: https://groups.google.com/a/chromium.org/g/blink-dev/c/OE1rXYv126c
Demo: https://document-picture-in-picture-api.glitch.me/display-mode.html
Bug:41493667
Change-Id: I16d0b7c62ed09220e4bbacdfa86191331f872d11

css/mediaqueries/display-mode.html

@@ -9,6 +9,7 @@
   query_should_be_known("(display-mode: browser)");
   query_should_be_known("(display-mode: minimal-ui)");
   query_should_be_known("(display-mode: fullscreen)");
+  query_should_be_known("(display-mode: picture-in-picture)");
 
   query_should_be_unknown("(display-mode: 0)");
   query_should_be_unknown("(display-mode: none)");