headless: make initial screen/window sizes configurable.

headless/lib/browser/headless_screen.h

Index: headless/lib/browser/headless_screen.h
diff --git a/headless/lib/browser/headless_screen.h b/headless/lib/browser/headless_screen.h
index f0c363c54f67427538280ca09d7b18ef35487490..962fc9c61b83c6efdccf5a5776eac74892df90b9 100644
--- a/headless/lib/browser/headless_screen.h
+++ b/headless/lib/browser/headless_screen.h
@@ -27,8 +27,7 @@ namespace headless {
 
 class HeadlessScreen : public display::Screen, public aura::WindowObserver {
  public:
-  // Creates a display::Screen of the specified size. If no size is specified,
-  // then creates a 800x600 screen. |size| is in physical pixels.
+  // Creates a display::Screen of the specified size (physical pixels).
   static HeadlessScreen* Create(const gfx::Size& size);
   ~HeadlessScreen() override;