Implement window management devtools commands for headless.

headless/public/headless_browser.cc

Index: headless/public/headless_browser.cc
diff --git a/headless/public/headless_browser.cc b/headless/public/headless_browser.cc
index 9be5aa50a1e906da78f816cc338897efebf17de1..7dbd7f41ea3c64c9850fe7f8d275bee05acaa2cb 100644
--- a/headless/public/headless_browser.cc
+++ b/headless/public/headless_browser.cc
@@ -22,6 +22,7 @@ namespace {
 // Product name for building the default user agent string.
 const char kProductName[] = "HeadlessChrome";
 constexpr gfx::Size kDefaultWindowSize(800, 600);
+constexpr gfx::Size kDefaultScreenSize(1920, 1080);
 
 std::string GetProductNameAndVersion() {
   return std::string(kProductName) + "/" + PRODUCT_VERSION;
@@ -47,6 +48,7 @@ Options::Options(int argc, const char** argv)
       product_name_and_version(GetProductNameAndVersion()),
       user_agent(content::BuildUserAgentFromProduct(product_name_and_version)),
       window_size(kDefaultWindowSize),
+      screen_size(kDefaultScreenSize),
       incognito_mode(true),
       enable_crash_reporter(false) {
 }