headless: make initial screen/window sizes configurable.

headless/lib/browser/headless_screen.cc

Index: headless/lib/browser/headless_screen.cc
diff --git a/headless/lib/browser/headless_screen.cc b/headless/lib/browser/headless_screen.cc
index e7be71a515bc0dcd4cb19b49a16d1f87a963bf0e..c8be18bfdb1804dab9c759e8c04d70a924dcdd2b 100644
--- a/headless/lib/browser/headless_screen.cc
+++ b/headless/lib/browser/headless_screen.cc
@@ -30,8 +30,7 @@ bool IsRotationPortrait(display::Display::Rotation rotation) {
 
 // static
 HeadlessScreen* HeadlessScreen::Create(const gfx::Size& size) {
-  const gfx::Size kDefaultSize(800, 600);
-  return new HeadlessScreen(gfx::Rect(size.IsEmpty() ? kDefaultSize : size));
+  return new HeadlessScreen(gfx::Rect(size));
 }
 
 HeadlessScreen::~HeadlessScreen() {}