Window Placement: Make the FullscreenOptions.screen non-nullable

Suggested via https://github.com/webscreens/window-placement/issues/13

Marking the screen member non-nullable allows callers to omit a value or
pass undefined to effectively request fullscreen on the current screen.
Passing null now yields TypeError "member screen is not of type Screen"

This also fixes a crash checking for the id of a null screen.

Bug: 1084321
Test: enable-experimental-web-platform-features and requestFullscreen({screen:null})
Change-Id: I92b7bd8367be1bafd8903c60a719ee45738a925d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2241729Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779570}

third_party/blink/renderer/core/fullscreen/fullscreen_options.idl

@@ -14,5 +14,5 @@ dictionary FullscreenOptions {
   FullscreenNavigationUI navigationUI = "auto";
 
   // https://github.com/webscreens/window-placement
-  [RuntimeEnabled=WindowPlacement] Screen? screen;
+  [RuntimeEnabled=WindowPlacement] Screen screen;
 };