Implement window management devtools commands for headless.

headless/lib/browser/headless_browser_context_options.cc

Index: headless/lib/browser/headless_browser_context_options.cc
diff --git a/headless/lib/browser/headless_browser_context_options.cc b/headless/lib/browser/headless_browser_context_options.cc
index e31158bdc4c70e88e66666ba15d86d1df9d0ca83..98ab2eafe67d9933b25b59f24a906b230a8a12f6 100644
--- a/headless/lib/browser/headless_browser_context_options.cc
+++ b/headless/lib/browser/headless_browser_context_options.cc
@@ -58,6 +58,10 @@ const gfx::Size& HeadlessBrowserContextOptions::window_size() const {
   return ReturnOverriddenValue(window_size_, browser_options_->window_size);
 }
 
+const gfx::Size& HeadlessBrowserContextOptions::screen_size() const {
+  return browser_options_->screen_size;
+}
+
 const base::FilePath& HeadlessBrowserContextOptions::user_data_dir() const {
   return ReturnOverriddenValue(user_data_dir_, browser_options_->user_data_dir);
 }