headless: make initial screen/window sizes configurable.

headless/lib/browser/headless_devtools_manager_delegate.cc

Index: headless/lib/browser/headless_devtools_manager_delegate.cc
diff --git a/headless/lib/browser/headless_devtools_manager_delegate.cc b/headless/lib/browser/headless_devtools_manager_delegate.cc
index ee5cfca7f1b511ec0d078eae879d0aebe6018850..4d9c8e5cc096fad3ebe0b9af3fa4adf1107a184c 100644
--- a/headless/lib/browser/headless_devtools_manager_delegate.cc
+++ b/headless/lib/browser/headless_devtools_manager_delegate.cc
@@ -59,8 +59,8 @@ std::unique_ptr<base::Value> HeadlessDevToolsManagerDelegate::CreateTarget(
     const base::DictionaryValue* params) {
   std::string url;
   std::string browser_context_id;
-  int width = 800;
-  int height = 600;
+  int width = browser_->options()->window_size.width();
+  int height = browser_->options()->window_size.height();
   params->GetString("url", &url);
   params->GetString("browserContextId", &browser_context_id);
   params->GetInteger("width", &width);