Comment that display::Screen::GetScreen() may return null

This came up in a recent CL review calling this function:
https://chromium-review.googlesource.com/c/chromium/src/+/2628705/6..10/content/browser/renderer_host/display_util.cc#b51

Bug: 1169327
Test: None
Change-Id: I33bae732cddb7345153f993b840dee29d22b2c49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2644109Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Auto-Submit: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845996}

ui/display/screen.h

@@ -41,7 +41,7 @@ class DISPLAY_EXPORT Screen {
   Screen();
   virtual ~Screen();
 
-  // Retrieves the single Screen object.
+  // Retrieves the single Screen object; this may be null (e.g. in some tests).
   static Screen* GetScreen();
 
   // Sets the global screen. Returns the previously installed screen, if any.